I've been using a 16qt for about a year and a half. It holds 4 meals right now (massive eating style with plenty of fruits and veggies ~600-700 cals/meal)
I've gotten used to it, but I still get plenty of looks. The most common comments are "is that full of beer?", "going on a picnic?", and my favorite "taking your fridge to work?"
mines an igloo max cold, it's held up pretty well. I find I do need something bigger if I'm going to be out of my house for more than 10 hours, but I've just been putting the dry stuff in a plastic bag or taking one of those "regular" lunch coolers.
However, I think anything bigger than a 16qt would be quite cumbersome, it wouldn't really hang at your side anymore, and it you put it under your desk you wouldn't have room for your feet.