Having gone from my OG TF101 to a nexus 7 (and unfortunately a kindle fire HD along the way), I couldn't see ever using a 10" tablet again (that is, for a non-productivity device). 7"(ish) form-factor is perfect IMHO.
Now, if we're talking about a full windows OS in a tablet with decent performance and usable battery life, then we can talk 10" (and a dock, of course, with a secondary battery).