Hello and welcome!
I would take the bracket off the bass drum and take it to a hardware store to find out the correct screw size. It will certainly be metric, probably 6 mm is my guess.
One word of caution: If you are planning on putting anything bigger than a small splash on that arm, I would recommend that you switch out brackets to something that has an eye bolt going through the middle that you can tighten down with a wrench. While you have the bracket off, measure the spacing in between the holes, center of hole to center of hole. Most of the older Pearl brackets had 1 7/8" spacing. If that's the case, than a Ludwig bracket will fit perfectly, as the hole spacing on that is also 1 7/8". The Ludwig part number is P1216D and they are pretty widely available. You may have to widen the holes slightly, but the security of an eye bolt that can be tightened with a wrench is worth the peace of mind. The Ludwig bracket comes with a large wingnut, you can just replace that with a 5/16" nut.
The reason I say that is because when just a small screw is tightened against the L-arm, it's impossible to get tight enough to prevent it from slipping when the weight of a cymbal is on it. Unless it's a very light splash, the arm WILL move, no matter how much you tighten it down.
Good luck.
