Generally the really small wheel folders are only for short rides. The 12" folders will not have wide enough gear ratios to do much more than short rides.
20" Wheels are the most common size on folding bikes. They offer several advantages over 16" and 12" wheels. First off there are millions of 20" wheeled bikes sold every year so tires and tubes are available almost everywhere.
Large wheeled folders often will ride just like a comparable non folding bike with the same wheel size. Folding frames are a little heavier and to get comparable components you will spend $100 to $200 more for the folding bike.
To the uninitiated a folding bike is just a folding bike. However from my experience price often makes a huge difference in the quality of the bike.
Gearing is a very critical factor for me in a bike. Gearing on small wheeled bikes is often narrower range than on full size bikes because of the limited space.