transformer watt meter for 120 volts ac

1/2A would be fine. I think I used a 1A as that was the smallest 120V AGC fuse I could get readily at the time. The goal, of course, is simply to protect from a fire or similar unpleasantness should the meter or voltage sense/power supply wires somehow develop a short. The supply wires can't be too big and still fit in the dinky little binding posts on these meters.

The meter itself probably uses something in the vicinity of a few milliamps, at most, I would guess. It would be hard to get a 120V fuse that is too small without it being a somewhat exotic part.