Yoni, I assume you're asking this question because you don't own a socket set and you are looking to purchase the correct sized sockets?
Typically you would pull out your socket set and fit the different sized sockets on the bolt heads until you find the correct fit. If you don't own a socket set be sure to get 6 point sockets, in metric size, that will contain the usual sizes. Standard socket sets will usually contain multiple sockets starting at either 8 or 10 mm going up to 19 mm.