There are many different wheel sizes available for bicycles nowadays and the general bike market leans towards the bigger sizes. Unlike the world of recumbents. There are several reasons why you can hardly find any 29” wheel recumbent bike or trike out there. At least when talking about recumbent cycles which are not supposed to be fast road cycles. Let’s go through all the possible sizes and mention their pros and cons. 
Be aware the wheel size is marked not only in inches but also in millimeters which is much more precise number, but be also aware that there are two quite different wheel sizes marked as 16” or 20” and they have different diameters actually. If you want to dig deeper into this topic, read the article on the very famous Sheldon Brown website here: 

12” wheels

12” wheels are used only on some children trikes, mainly the KMX where they are used as front wheels. 

16” wheels

16” is a very small wheel size and is used on some children trikes, but also on some homebuilt recumbent bikes mainly those which main intention is to be folded. Folded very small. The size most of the people understand as 16” is 305 mm, but you can also find a 337 or even 349 mm wheel size marked as 16″. The latter one is used on some Greenspeed trikes. I have to mention the Greenspeed GT3 which is an amazing little trike that folds very small, has great steering and the turning radius is just breathtaking. All that also thanks to the 16” wheels, but mainly because of its great designer – the founder of Greenspeed – Ian Sims who unfortunately passed away in 2019. 
In general, the advantage of the 16” wheels is their compact size, and the disadvantage is mainly their lower comfort. The small wheel simply gets deeper into the pothole and is also much stronger which makes it less comfy. That’s why they are pretty often used with some balloon tyres for increased comfort. 

20” wheels

20” is a very common size in the recumbent industry. Such wheels are used as front wheels, but also rear wheels on two-wheelers and also as the main wheel size for trikes. If we talk about SWB two-wheelers, the 20” front wheel gives the rider lower sitting position, but still good speed. In the trike world, this size is popular mainly because of its stiffness as it withstands the side forces very well. It also doesn’t require so much space around it when steering while being used as a front wheel. The 20” wheels give any recumbent cycle a compact size, but good overall characteristics. 
The main size in millimeters is 409, but you can also find some 451 wheels which are often used in Southeast Asia and Japan or as a larger thus “faster” size. It is much harder to find appropriate tires and tubes for 451 mm size and also the choice is very limited. 

24” wheels

24” wheels are considered by many as something between the 20” and 26ers and are used where the larger ones are too big. Some two-wheeler producers were offering this size in the past like the Challenge, Flux or AZUB, but only the german Flux is offering it nowadays on their S900 model. Among trikes, you can find it mainly on the Terra Trikes. Actually, their new Rambler (2020) has 24” wheels all around in its standard version and also the Rambler All Terrain has them as well. 
In general 24” wheels give recumbent trikes higher sitting position and usually also better ground clearance, but are not as big as 26″ wheels. On two-wheelers, they are liked by smaller riders who would love to have some large size wheels, but are too short to ride a bike with them. The disadvantage is that it is kind of hard to find good quality components for them like rims, tires and tubes and also the overall choice is somewhat limited. Seeing them as some awkward size which you can’t find in a classic bike shop wouldn’t be correct as well, because they are often used on a children bikes. 

26” wheels

26” is the most used size among the “large size wheels” in the world of recumbents. You can often find it as a rear-wheel on both the trikes and two-wheelers, but from time to time you can see a bike or trike with all wheel 26″. Let’s say they are large enough, but also small enough. Having a two-wheeler with both wheels 26” makes it look nice, but also ride somehow smoother and faster, but it can be a challenge for some shorter riders as the seat is way too high on the short wheelbase bike. Among the trikes, it is the most popular rear-wheel size as people tend to think the larger the better. It is not like this with trikes and I will go over this topic soon, but let’s say people like it most.  When talking about LWB bikes the 26″ wheels looks great, rides great and you can’t really find any disadvantage on them.
There are also trikes with all three 26” wheels which are mostly the fat versions, but AZUB, for example, makes two of their models as 26ers. The thing is the trike is quite big then, and although it has better ground clearance and you can use it on gravel roads or jeep tracks, you have to consider the size when storing or transporting the trike. Also the weight rises up somehow.  
Unfortunately, the 26” tire size is getting somewhat old in the world of upright bikes which lead to the fact that there are still fewer and fewer components for this wheel size. It is still much better than with 24” or even 20” wheel size, but the 29” has been taking everything and producers have been limiting their choice of components for 26” wheels since the 29ers became popular. We will see how that will develop in the future. 

27,5” / 650c wheels

Bacchetta Carbon Aero 650c
27,5” wheel size was very popular among the MTB riders around 2017, but in 2019 the situation has changed and it seems it was just marketing hype and they are slowly disappearing. You can’t really find any recumbent cycle with this wheel size. Well, you can, as 27,5 is also known as 650c which is a size used on road bikes so some light and fast two-wheelers use it in case the 700c is too big for some shorter riders. You could find it on some Bacchetta recumbents for example. 

28″ / 29” / 700c wheels

28”/29”/700c are all three basically one wheel size only but are used for different types of bicycles. The 28” wheels are often used on cross or trekking bikes with narrower tires, 29” are coming from the mountain bike world and 700c is a road bike size. Although this size is very popular in the world of upright mountain bikes and also road bikes, in our recumbent world they can be found only on fast two-wheelers. Trikes have them as a rear-wheel only if they are supposed to be used on roads. Why is that? Because the larger the wheel is the weaker it is and the worse it withstands the side forces coming from going fast through curves. If it would need to withstand not only the side forces but also the forces coming from the terrain, it would survive much shorter time and it would also put way more stress on the trike frame. Compare to upright bicycles the trikes are also kind of larger vehicles and larger wheels make them even bigger especially when you have them suspended. When talking about two-wheelers which would be used for off-road riding or common leisure rides, mainly the short wheelbase one (SWB), the larger wheels would increase the seat height to a level where it would be very uncomfortable to operate the bike even for very tall riders. Not talking about a full suspension two-wheeler which would be sky high with 29” wheels. This is mainly the reason why recumbent manufacturers stick with 26” wheel for most of their models. 
And this is it. Please note that I have tried to write this article as a preview of the sizes and I have tried to make it as simple as possible. There are many other posts here and there that describe many other aspects of different wheel sizes. If you have any comments or suggestions, feel free to write them hereunder. We all will be more than happy to read them. Thanks!