This is what I have:
I don't have a proper measurement tool to measure its diametar precisely, but it's slightly over 2 mm. Note that this type of screw might be unsuitable for a laptop cases (because of its large head). The best thing to do is to go to a computer store and explain there what do you need, they might have the best fitting screws.
Also, on your second picture, the hole next to the ram is probably for the back cover screw.