Nellie, the head of the screwdriver should be as wide as the slot in the screw head. It can be little wider if the screw is not recessed in to the top of peg. If it is narrower, you can still use it, but it is more likely to damage the screw head. I am guessing that you have mechanical friction tuners that use a flat head screwdriver. If you have open geared guitar tuning machines, you will need a Phillips screwdriver. In that case you will need a #2 Phillips head screw driver.
Ken
"The dulcimer sings a sweet song."
