How to select an ideal array size in C?

So far I've seen so many pieces of code where arrays are declared with a seemingly random size, I mean, the size doesn't seem to have a specific reason, but it seems more like a magic number. How do I select the size for my arrays? Is it just about putting on a big number? I still don't get it, and I feel like if I knew that, my code would get a lot better. Thanks in advance.