I'm 6 feet tall and a four foot pad still supports my knees when I'm on my side which seems to be the key. If my knees were off the end it would probably be uncomfortable. A couple of things that can make a difference:
1) Do you put your pillow on top of your mat or off the end? If on top you lose length and your knees might drop off the end.
2) Do you plan on putting anything down on the ground under your feet?
Your legs won't be 2.5 inches above the ground anyway because the pad compresses. Most Air pad users fill their pads up full and then deflate them a bit after lying on them (a comfort adjustment) so they tend to stay soft and compressible.
I also carry a .5 inch thick foam bum (seat) pad that I use under my feet at night. That makes a big difference in comfort and especially warmth. You can really feel the cold if you don't put something under your feet. I tried using my pack but it didn't stay under my feet as well as the bum pad.