I am 5'4" and use the x-small thermarest prolite (it is mummy shaped) and also bring a 2'x2' cut square of the blue foam pad (sold at REI). I use my pack and extra clothing as a pillow, shoulders to mid-thigh on the thermarest and feet on the blue pad. I like a separate foot pad because I curl up (sort of jack-knife)and like to be able to put the foot pad at any angle. The foot pad also doubles for my "sit pad" to use outside the tent. The prolite weights 8 oz and the blue square 2.5 oz. This set-up is OK for summer, but not enough insulation for winter (not for sleeping directly on snow).

However, when I use my bivy sack, I find a longer rectangular pad works better. I then use a 3/4 length thermarest (14 oz). I still can use the pack for a pillow.

If you are choosing between a pad "exactly your height" of one foot longer, I see no reason to get one foot longer. In fact, if you do not put your head on the pad then one foot shorter is OK.