shoe size. LOL.
Make a note of every single thing they do that seems weird or bad.
If the tables are full of terrible players, make a note of every single thing they do that seems competent, too.
Make a note of every hand they show down that you wouldn't have played preflop in their position.
Every second you're not making a decision about a hand you're playing, be watching what the other guys are doing, and make a note about it.
So yeah, if you notice a weird shoe size, definitely write that down.