I don't know how to cycle for long because my back will pain, arms go numb and the nut cannot take it.
That's why folks go for bike fitting. If for foldable bikes, then LL, can do some stuff like riser bar to adjust reach but that's about it. There's no bike frame size to choose one.
Only thing can do is train upper body and back muscles esp the erector spinae muscle (so-called lower back pain which others always complain about) , which i am doing right this very moment.

Coz their core is bloody weak. Actually for runners too, is good.
Hemce, since last year Sep when i started cycling, i never had any neck pain arm pain back pain, zero episodes. Even tho i spend so many hours (approx 20 hrs) on the bike per week. Nothnig of that sort has bothered me, coz before i started cycling i have already started strength training in many areas.
Actually i train strength daily, but alternate days is hit different muscle groups.....once in a while i take 1 off day.
No sweat one, esp for 1 day the push-ups i do in server room (cold mah),
Leg muscles more sparsely, like once or at most twice a week with bands and in conjunction with plyo (single leg pogo mostly), but mostly calfs, glutes and hamstrings only. Calfs i try to do in server room too, the single leg weighted calf raises and single leg pogo jumps.
Now listening to this while training..... shiok.