The general wisdom is to go 2 times your batch size, and perhaps a little more.
I do 3.5 gallon batches in a 5 gallon pot. To get away with it I have to sparge to get up to pre-boil volume, otherwise the grains + water will not fit. My limiting factor is really pre-boil volume. I start my boil with 4.25 gallons of wort and boil down to 3.5. I am really pushing it when I do it this way (I push the edge on boilover), and plan to up size soon.