Maybe one with a blade that's, say, 3 - 4 times the width of eyeglass screwdriver blades. If it fits into the hole in the carb casting, it'll probably be the right size. Get a short one, since there isn't much room under the carbs to move around. The screw isn't hard to turn. BTW - I'd suggest wearing some sort of heat proof gloves, since you'll be making your adjustments with a warm (hot) engine. I use an old pair of summer weight riding gloves.
Something doesn't sound right, here. I used a small screwdriver--not a tiny one or anything special. Are you sure you are in the correct location. Your '01 bonnie may have an anti-tamper plug (shiney cover about 10mm in diameter with a tiny hole in the center) covering the pilot (idle) adjustment screw.
The pilot screws are located at the very front of the carbs next to the carb heater wire connections. It sounds to me like you are trying to put a screwdriver into the tiny hole that is behind the heater wire connections--this is not the correct location for the pilot screws! Look at the carbs from underneath with a mirror. The pilot screw channel is located at the very front of the carb and to the outside of the carb heater connections on the right-side carb.
If yours has the anti-tamper plug covering the pilot screws, drill the tiny hole in the center of the plug a little larger--I did this by hand with a small drill bit...then screw a "grabber type" screw into the hole and use pliers to remove the plug and you will see the pilot adjustment screw. Repeat for the left-side carb and you are ready to try your hand at adjusting your pilot mixture. :-D
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