hello and question
 

Im just a casual biker and mainly commuter to work. i went to a store and they had a rear bike rack for $5 because it was missing the hardware, no screws. Now i was wondering what size and how many. Kinda an universal idea, i just want to head to home depot. any help would be awesome!!

That's a tough one. If you post a picture we can probably help you. Or, you can probably bring the rack with you to home depot and the hardware guys should be able to help you.

I think an M5 x 20mm will work on the chain stay, and a slightly shorter M5 on the seat stay. Unless you have a seat post rack w/o lower struts...

isnt everything a 1/4 inch? Same size as your standard bottle cage screw holes? More important is that it fits into the frame. Would one want washers in there anywhere?

Water bottle mounts are M5 (5mm x 0.8mm), as are most dropout eyelets. Washers under the bolt heads is good practice.

Most bike hardware is metric, especially the smaller screws/bolts.

Nothing against Home Depot, but if you have a smaller, independent hardware store nearby, and just ride over there, they might help you find the correct screw sizes.

Most bikes have 5mm threads on all the eyelets & water bottle holes. For a rear rack you want short bolts. About 10mm would probably work. Stainless steel bolts with allen heads are the best.