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, Sour Rye (w/BugCounty)
SecondaryFlanders Red, Frambrozen, Grimm V2
Bottled Burton Ale, Wet Hopped IPA, Roese Kriek, "Grimm" RIS 23%, Saison Du Flood,"Warrior" IIPA 17%, "Oaked Warrior" IIPA 17%
Kegged Hip Hop IPA, Chocolate Cream Stout