I had 4 gallons boiling (3 gal batch) in a 10 gallon pot and I had a big boil over. that just tells ya that you need to keep an eye on it regardless of pot size. but to add my advice to the op's question. I usually start with 6 gallons in the boil and expect to lose half a gallon during the boil and another half between trub, transfers, and spillage during botting. that's how i try to end up with 5 gallons finished beer. so If i were you i'd boil around that much if you're boiling for a full hour. full boils are really only gonna help.
PrimaryLambic (WYeast blend), Local Lambic (spontaneous fermentation), Flanders Red with ECY02
SecondaryFlanders Red, Oud Bruin,
Bottled Burton Ale, Wet Hopped IPA, Roese Kriek, "Grimm" RIS 23%, Saison Du Flood,"Warrior" IIPA 17%, "Oaked Warrior" IIPA 17%
Kegged Little Bastard IPA 3.5%, Top Secret Barleywine aged on vanilla and oak