Since It's possible to place a bone wherever you want on its parent, what kind of effect does the size of a bone have? Is it only for visual guidance?

You mean bone length? It is only for your visual convenience in the editor, though the length is available at runtime. Eg, you could use it to compute the bone end location. But yeah most of the time it doesn't matter.

Good. thanks. Only wanted to know If I could mess up anything.