not sure if headspace is an issue, but I just did my first AG brew today and as soon as my wort started to boil, boom massive overflow, so I may look into a bigger brew kettle, mine is 5.5 gal now.
I brewed quite a few batches in my 7.5 gallon turkey fryer. Got a few boilovers and eventually started using gas drops to control the foam and it worked much better.
But I wish I had gotten a 10 gallon or bigger at that time. Except for the fact that it was easily the cheapest way for me to get a burner and kettle that worked, I can't say much else for it.
If you don't need a burner and you are only looking for a kettle, I highly recommend you consider at least a 10 gallon. Maybe even 15 if you are looking to do 10 gallon batches in the future. Almost any kettle will work as long as it's SS or aluminum.
Thanks for all the input. I think I will go with the ten gallon in case I move to bigger batches and to ensure my dog doesn't have to lick up that much wort.