Unless it's leaking of you HAVE to remove them I wouldn't touch it.
If you just have to remove them, try to get a measurement of the actual threaded portion of the bolt before you try to remove them so you will have them on hand.
Just a guess butI'd say 10mm or 3/8". Go with stainless fasteners and they won't get nearly as bad as these are. And since it is only a couple the price won't hurt your pockt.