How to identify a nut
A few alternative approaches that don't require guessing:


If the nut fastens to a removable bolt rather than a bracket with a permanent threaded rod, don't bother trying to match the nut.  Replace both the bolt and nut with something about the same size that fits the existing holes.  
If there is a removable bolt, but it also threads through other hardware, temporarily remove the bolt and nut and measure the size using the bolt.
If there is not a removable bolt, temporarily remove the nut and measure the size and thread of the exposed portion of the "bolt".  Put the nut back on until you buy the replacement.  If you have a thread gauge (or there are cheap ones you can buy), that will make it easier to get an accurate measurement on either the bolt or nut.
If you don't have a thread gauge and measurement is difficult, temporarily remove the nut.  Lubricate the nut and threads, then make a quick "casting" of the thread with hot melt glue or other material, including enough material to grab to unscrew the casting.  Unscrew the casting and put the nut back on the door.  Then take the casting to the hardware store to find a matching nut.