5 gallons in a 55 gallon boil kettle would only be an issue if the kettle were really, really wide. My BK is 25 gallons and is pretty wide (2.5 feet) and I have no issue doing 5-10 gallon batches. I wouldn't get a kettle that large unless I was going to get some use out of it. It's a pretty expensive piece of equipment, bulky to store, impossible to lift when full of liquid. The only other (small) issue I could think of is the mass of the kettle absorbing heat. I'd imagine it would take a little more time/fuel to bring 5 gallons to a boil in a 55 gallon kettle than in a 15 gallon kettle.
"Good people drink good beer." -HST