For a tri-fold from Brompton or a clone, actually the wheel size doesn't change the size of the frame, 20" and 16" have similar dimension when the bike is unfolded. Your wife will experience more or less the same position on a 16" and 20" clone with the same type of handlebar.

It only change the folded size and how the bike will roll on bad roads or trail where a 20" will perform better if it has wide tires (50mm or wider), 16" Brompton and clones have 35mm wide tires since their frame doesn't accept wider tires and anyway there are no wide tires in 16"/ETRTO349 (Brompton and clones 16" wheel size means ETRTO349, not the usual 16" = ETRTO305).

Note that many 20" clones do not accept wide tires, only recent one cloning the G-line accept 50-55mm wide tires like the G0line does.