Is the answer determined by the rider's height, leg & arm length, and how well they fit onto the frame design?

The frame is made for the rider size not and made to work with the wheel size chosen. Roger

Nb:
 

Folding bikes are 1 size products, Though Brompton has a couple bar and seat mast options..

Only bespoke ordered Bike Friday offers the buyer 'sizes' ... Length and heights.

20 inch just because that is the most common and parts are readily available.

I never fold the 26" Montague (or Raleigh 20 for that matter). But come spring I'll have a Vilano Urbana (20") in the trunk of the Del Sol and a Dahon FS9 (20") in trunk of Accord.

Seriously considering the DownTube Mini Chain Drive. Want, want. Gear inches are SO appealing. But, no way can I justify it.

I need to plan some flights someplace.

I'm still a student on this topic, though my understanding at the moment is that wheel size and tire size / type is determined by the application and the terrain.
I also learned that the wheels and tires themselves determine the level of comfort, for on-road riding, more than suspension and frame materials.
The wheel diameter, as I understand, mainly affects the stability, handling character, and of course the folded size.
Still learning.

Note: whatever you buy will need to be customized, tires, gearing, seat, seat height, etc. Suggest you reach out in your community to other folder owners. We love to do show and tell.

fatigue question
 

i'm 66 and want a foldable bike to carry some, and i want to ride about 40 to 60 minutes a day or so. there will be hills , my question is which bike under $600 to buy and what size wheel 16 or 20, is there a fatigue factor after so many miles or time between the two. example after ten miles one would be tired on 16inch wheels but not the 20 or do you think 16 ok at that distant? I'm looking dahoe and citizen right now and this will be my first folder and probably only one so name one please, used bike is out of the question.... thank you david i own a regulat bike but want a folder to drive somewhere else thanks again

If you live in a metropolitan area put a call-out to people with folders. Most of us want to show them off anyway. Note: very few shops carry multiple folding bike manufacturers.

24"...just because Helix can fold quite small for a 24". I've not looked at a lot of folding bikes but my impression is that it folds smaller than most 20" bikes so why not have a larger wheel. And it rides like a regular bike. :)