Anyone know what size hexhead is need to open the silver screws on the inner part of an xbox?

I recently found out how to fix my old broken xbox. My new one has some problems and I would like to repair my old one to use untill I send my new one in. The only problem is I cannot find a screwdriver that will work on the inner screws. I have gone to two different hardware stores, but cannot find the right size. Anyone know what size I need?