S&W will send the wrench at no charge, at least they did for me. I do not remember what size it was, some one will come along with the proper size, once I had that I also ordered one off of ebay.
If you bought it new and can't find the wrench in the box, look carefully in the plastic zip lock bag that came with the gun. Also consider adding a little Loctite Blue to the screw threads so the screw won't back out while in use (a common problem that disables the gun until fixed).
My laser cover screw was backing out after firing about 50 rounds. I noticeed the loose screw before it backed out so much that it jambed up the slide. I put a tiny drop of blue loctite on the screw and this prevented the losening of the screw. However I cannot now remove the screw at all and have stripped the head of the screw. I'm thinking of sending it S&W for repair. Have I voided my warentee with the loctite. I've not used S&W customer service befor. Am I going to have any problems?