Usually all the necessary screws should come with the case. Also each case should have a manual that lists all the available components including the screws. If you don't have that you should try each screw size and use the type that fits in each component. Usually the smaller screws should be for the SSDs, the next size for the HDDs and the bigger ones for the expansion cards and the PSU. Finally the bigger ones may be for the case side panels. Also keep in mind that each PC components may come with its own screws. PSUs for example do come with their own screws just in case your case doesn't have them.