I have a Thermarest 'trailcomfort' which is the slightly thicker version (5cm) which seemed a lot more comfortable on the shop floor than the 'lite' version.
I bought the full length version for comfort/thermal insulation from the ground (I have problem knees too), and as Flyingdoctor suggests, if you fold them in half lengthways before you roll them up they pack away really small... same size as my 3 season sleeping bag in a stuff sack.
One thing I would say is that Thermarests are quite narrow - it can feel like you're sleeping on a (albeit comfortable) plank... They do do a wider version (in America at least) which is nice if you sleep on your back/have big shoulders?
xxx
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