The main size adjustments for most folders which have only one size frame per model are :
Seatpost, often telescopic
Handle bar stem, can be telescopic also
Fore and aft seat placement. Bromptons had a seat post device that allowed for more front to back adjustment.
Brompton's Pentaclip can be flipped to lower or raise the saddle.
Devices such as an Aber Hallo can extend reach or height of handle bars.
Handle bars can be used to raise the grip area or to place hands farther forward or farther behind the factory set place. Bar ends can be used with flat or riser bars to extend the reach or provide alternate hand positions.
However, it is good to remember folders also have weight limitations. Bike Friday makes special folders that allow for much heavier riders. The weight limitations can also include the weight of cargo, as well as the cyclist.
My experience has been mostly with my Xootr Swift and my Brompton. Both of these can fit very short folks and people over 6 feet although what the body proportions are is a factor. Some people, including myself, like to have a folder around in case of guests who might like a ride since they are so adjustable. To be honest, the primary reason I have them is I love to ride them.