I was asked which winch I was planning on using on my winch bumper. That might be the best place to start. Determine which winch you are most likely to use then go to that site and get the dimensions. True they should all be the same size but at least this way you'll be sure what you are building will fit the winch you will be getting.

Alternatively you could pick a couple of winches you like then use the largest sizes of the two, three, etc...

Of course just buying the winch first and building around it would work.