I just now stumbled across this thread while trying to find the answer to a slightly different question regarding changeling height. Mainly, how it is affected by the race of the father. After all, 4'4" to 4'10" would be tiny for an Elf, but freakishly tall for a Halfling.
Didn't really find the answer I was looking for, but the best house-rule I've been able to come up with for it could apply here:
Roll height and weight as for the race of the father, then add an additional height modifier. Say, 1d4 for Small races and 1d6 for Medium. Leave weight unmodified, for the 'slender' part.