When I play Minecraft I can't switch my mouse to the second screen (if I'm in the inventory or in the menu), what should I do?
Are you in full screen? If so, then turn it off.
But then Minecraft is mega small or not?
No.
I don't play Minecraft in full screen mode but only in maximized window mode. You hardly notice the difference and it is much more pleasant if you want to quickly switch to another monitor, because you don't always have to go out of the fullscreen.