There are several different wheel sizes that are common with Folding bikes.
12" While rare there are a few 12" wheel folders. They tend to be very inexpensive bikes with one speed.
16" There are a lot of 16" wheeled folding bikes. Europe's largest manufacturer of folders (Brompton) only builds 16" folding bikes
20" folders are probably the most common for several reasons. The folded size of most 16" bikes is just a little bit smaller than most 20" wheeled folders. Derailleur gearing is much easier to use on 20" wheels because with 16" wheels the lower pulley ends up very close to the ground.
24" Folders are fairly rare. The wheel size is not very common and the folded bike will be barely smaller than a 26" wheel folder.
26" Folding bikes are fairly common. You will easily two bikes into a station wagon---maybe a bit big to fit one into the trunk of most cars.
27" Folding mountain bikes are made by Montague. I actually recently bought one and I have modified it significantly. I like it a lot after a few mods.
There are significant differences between the uses of different wheel sizes. The tiny 12" bikes and most 16" normally are for very short rides and or short commutes. The 16" Brompton is an unusual in that it is more multipurpose than most 16" folders.