I found this size box fits my letter-size documents without a problem. I primarily use this fire retardant box as a box within a big safe as added protection for important original documents.

I thought carefully about which of these metal lock boxes to purchase. This box is fire-retardant, not fire proof, but it is much lighter as a result. It is very likely that I will put it in a safe deposit box, so fire proofing is not my first concern. The size is perfect for storing important papers flat in their file folders (or envelopes for warranties, etc.). The height of the box is 4". It will slide easily into a safe deposit box; and it has handles on each side to make carrying it easier. Last, it was less expensive than the other boxes. I don't imagine it would withstand anyone trying to break into it, but I doubt any of the other boxes would either; and my goal was more to get all my important papers in one place, so I could grab them in case of emergency, or easily transport them somewhere else for safe storage.