Trying to keep the overall diameter of the replacement tire the same
as for the orginal equipment tire isn't absolutely mandatory. Some
pilots use taller tires to get the back end of the trailer up higher
for more clearance when pulling out of a driveway, though that could
complicate rigging the glider with the trailer still hitched to the
tow vehicle. Other pilots swap the wheels for larger, wider ones and
mount low aspect ratio tires for more sidewall stiffness and
(hopefully) more stable towing, although this combination can give a
harsher ride. One of the most fundamental factors if you're keeping
the original wheels is wheel size, specifically width. Lower aspect
ratio tires may require wider wheels. You'll have to check your
current wheels against the allowable spec for the new size tires to be
sure they're OK.

Chip Bearden
ASW 24 "JB"
USA