In addition to seconding the link Huckleberry posted, there is one absolute rule of safes:
Always buy one twice as big as you think you'll need. Always.
I'm sure that's mentioned many more times in this thread. It's so true and not always for more guns. I put in cameras, important papers, some old watches and valuable coins and boom...all of a sudden 1/4 of my safe was gone to non-gun stuff.