1 1/4" Precision bass screws. Warmoth's got 'em, they're all over Ebay, DiMarzio makes some.... I've never found hardware store screws both long and narrow enough. Depending on pickup height above the body, and how high the ears are on the pickups, you have to be excruciatingly careful (sometimes) not to poke out the back. Sometimes you have to glue little blocks in the holes, or sometimes you have to start the screw threads, then file 1/16" off the tip of the screws and repoint them with a triangular needle file.
http://www.warmoth.com/Bass-Pickup-Mounting-Screw-Doz-Chrome-P159.aspx