Speaking as someone who doesn't own a tablet, but will probably get one soon, I'd be inclined to get a 7 inch one for outdoor use. Just that bit more packable I'd guess from playing with them in shops, easier to hold for a while without straining your wrists (assuming there's nowhere you'd want to put it down).
I'm only holding off as there's apparently a new Nexus 7 model coming out in July, so waiting to see what it's got. The current one seems to come out on top for battery life.
I picked up one on Thursday and tried it out this weekend.
Smaller than my previous pillow, but firm, which I like. I did manage to ping it away from my under head a couple of times (think tiddlywinks) as I had it on a shiny reflective roll mat. Reckon a blob or two of seam sealant on the underside to make it less slippery ought to sort that.