There's no specific dimension / resolution.
However, if you want to make your own border and avoid edges being cut, you should choose a resolution that is dividable in squares, both horizontally and vertically.
You can always count backwards. Let´s say you want a puzzle that is 8x5 pieces big. Just choose a size for the square e.g. 150x150px. Then multiply it with the number of pieces you wanted and you'll have a resolution (in this case 1200x750) that will work without any edges being cut.

We don't have a feature for adding borders to workshop puzzles at the moment, but we will look into it and see if we can add one. :)