I always wondered what this thing was hanging in my parents garage - turns out it's a coal screen. I haven't used it to screen ashes for unburned coal but I did find a good use for it today.
Some of the nut coal I use includes dust (fine coal) at the bottom of a bag. I've been dumping the bottom of bags into one bag for about a month and today used the screen to make a nice 3/4 bag of clean coal.
The screen measures 14" in diameter (although I think making it the size of a dry wall bucket might work better) and 3 1/2" deep. The screen holes measure 1/2" square and are supported on the bottom by two wire rods.
This thing works great and if I have to clean out the stove, I'm sure it will screen ashes too.