I'm 6"5 and have long legs. My advise is to try to sit in as many
gliders you can - go to a contest or large club and start asking.
I have a thin parachute by Butler and wear wrestling shoes or anything
with thin soles (sneakers easily add an inch to your leg length).

My experience:
I flew an ASW-20 for a few years with a modified backrest comfortably.
ASW 27 is too tight, only just possible with a special cut out panel.
ASW24 can't even close the canopy

Ventus/Discus 2B is very comfortable. Lot's of leg room.

Currently I fly a DG808B. without backrest quite comfortably, but
little shoulder room.

Bottom line is that tall pilots have to compromise and will have to
suffer a bit more - and be prepared to modify and experiement until it
is just right.

Chris