I'm with Krush. Pen and paper.
+1 for me too.. what Krush said.
Also, to add to that,
If you have any type of "existing conditions" drawings that you can print to take with you, it makes it easier to measure and verify dimensions.
At the very least, you don't have to sketch everything, just what's missing, and jot down the field measurements right on the drawing.
11x17's if you can pull it off.