I'm looking at different options for certain posters where some will be edge to edge and others will require a border. A perfect example for illustration is the travel posters Tom shows which he might actually sell. So the main image is placed within the poster but has a lovely border around it, with the bottom either having a larger blank area for text or a same-size border around the entire image. This will also allow the customer to frame the poster and keep a lovely border instead of having to place a matt frame around the poster to get a border.
I don't really know the best border size, if you used the travel poster as the example what are those border thicknesses around the main image?