Lots of poeple do full boil bathes on their stove but most of them do smaller batches due to the lenght of time it would take to boil a full 5 gallons or that they have height limitations for kettles. The size of your kettle will depend on the size batch you do. I have a 30qt (7.5 gal) kettle I use on my turkey fryer and do full boils for 5 gallon batches. I would'nt go with any smaller of a kettle though because I like the amount of room left on top to help prevent boilovers. This same pot would never fit on my stove because the microwave/hood is placed directly above my stove and I wouldn't be able to take the lid off, stir the wort, or put in a chiller. If you are dead set on a full boil just make smaller batches. If you want to keep doing 5 gallon batches topoff with cold water, which would help cool it to pitching temps faster as well. Even when I do full boils for 5 gal batches I usually have to add a small amount of top off water anyway because of evaporation.