The general concensus around here is if you can do a full boil, then do it! if you can only do 5 gallon, that is still better than doing a 2.5 gal boil!
In order to do a full five gallon boil your kettle needs to be able to boil down anywhere between 7 and 6 gallons, down to 5. I have a turkey fryer kettle, and typically brew with a 6-6.5 gallon pre-boil volume, brewing between 60 and 90 minutes (if I ave a lot of pilsner, then I do a 90 minute.)