I'm 5'7" and still use a medium NeoAir. I sleep on my side, knees bent, and just fit on it. Putting the pack at the foot end gives me a place for my feet to land if I roll on my back.

I also use a BA Q Core - very posh, full length - and find that it is just a tad more comfy. Also it's rated for colder weather than the NeoAir.

The most comfortable sleep of all is in my hammock with an underquilt. I still try to use it when I can. The only reason to not be in the alpine...
_________________________
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com