Limewire itself is legal, how you use it may not be.
As far as making sure you "get the right file type and size" goes, that means diddly. Anyone can make any file seem to be an mp3 or whatever at the "right" size but it does not necessarily mean it is what it says it is. Many times I'e seen people download what is supposed to be a "legitimate" movie and get porn, malware can be piggybacked onto things. Limewire is notorious for that sort of thing, you are best to avoid it like you would avoid herpes (unless that's the sort of thing you WANT to catch)...