If the footstrap screw you have stripped is a normal strap screw (that's to say, NOT the FIN BOLT screw being used to hold a rear strap on Formula), then the advice linked by Phill104 above is the best advice.
Please check it and let us know if you have any extra questions.
NO - the best option is NOT to simply use a longer screw in the footstrap insert; using oversize (by length or diameter) can cause a split the insert - and cause a long term (and very difficult to repair) leak into the board.
Also the screws used in the footstrap insert are a special custom thread pitch designed especially for use in the "softer" plastic inserts (standard now for just about all windsurfers - but only in one length - and generally near impossible to buy in general use outside the sport). Using a regular pitch (finer) thread screw doesnt leave enough plastic material between each thread when the screw is inserted - and this is far more likely to strip out - and in a half or already stripped insert , the worst thing you can do.
Please check the other thread (the first half, that deals with footsrap inserts) - and please let us know if anything is unclear or more info needed.
Cheers ~ Ian