I spent a couple of years doing full boils in a 7.5 gallon turkey fryer. Figuring evaporation, I'd start with 6.5 gallons in the kettle. Fighting boilovers was tricky... I'm now in a 10 gallon kettle, and it's much easier to avoid them.
On a regular stovetop, you could do the 2.5 gallon, top up afterward process. But propane is fun.