Online framing companies are pretty good.
All you have to do is measure the area (length, width) of your art print. Take into account what you want on show and what, if any, you want hidden behind the border.
That was my approach to framing my Mr. Freeze
I looked on the Sideshow website for inspiration. The plan was to have the art on show, with a 10mm white border around the edge, and calculated length so that the signature at the bottom would be in view.
The black frame width is 4cm
The black border overlay is 5cm
White looks good. The overlaid looks odd in that shot, as the sides look narrower than the top and bottom?
It might be the result of lens distortion?