OP, I'd generally recommend against 24" wheels and tires as the tire selection is very limited, it's the one drawback of the Helix folding bike that only comes that way. 20" wheels are more likely to provide a bike that is better proportioned for your height than 26" or 700c wheels. I now ride a 20" wheel folder and it's now my full-time bike, by choice. Even though only one frame size, they often fit a wide range of people well, because on many of them (like mine), the handlebar post (very long stem) and seat post are not parallel, they diverge in angle, so as you adjust both upward, the effective top tube length automatically increases. However, Bike Friday makes bikes in custom sizes and I think 3 standard sizes, I have seen used ones in "small" frame sizes on craigslist at big discount from new because there is a smaller market. Just be aware, many are equipped with Internal Gear Hubs (IGH), I'd stay away from one of those used, impossible to check inside condition on a used one. The Zizzo bikes as mentioned above are great deals brand new. So are Origami bikes.