I note stack off ranges, weird lines with nut hands, how they play sets, responses in 3bet pots and anything else unusual. I try to include as much relevant info as possible particularly position. Stack size is not so impt, regs are usually always 100bb and fish dont adjust to stack sizes so it doesnt matter. I try to include any recent history that may provide a framework for the action noted eg if you 3bet a guy 3times in an orbit, and he stacks off light the 4th time, it probably doesnt mean he stacks off light in 3bet pots.
When playing a session, I often just mark the hand for review and take the notes later if it is a reg. If it is a fish i dont expect to see again, then i quickly scramble the note in whilst playing because it is more likely to come in handy straight away.
There is an excellent post by Zeth:
Note Taking