Pipe size?

I want to swap under sink valves for 1/4 turn values. I ordered the ring remover tool, but not sure what size pipe valves I should order. The only pipes I can actually measure are to the hot water heater and they're about 3/4 inch. Should I assume the house is all 3/4 or shut the water off, drain the hot water off/drain the tank and measure the actual valves pipes?