If you will be using a 16x20 frame which is considered a "standard" size frame. You would position the vertical so assuming a 1/4 " border above and on both sides you have enough room for your signature and some space below it and still have a longer mat on the bottom. The horizontal print presents a different issue. If you want the frames to be the same size 16x20. You would then have a 2 3/4" on top (1/4") white print paper and a 3" +(1/2) on bottom and then 3 1/2" +(1/4") on each side. Frequently photographs are signed verso and not dry mounted and matted right to the print. This makes a very nice presentation especially with an 8ply mat board. In that case the color of the mat which may not be the exact matching white to the base white of the print. When possible I have stopped dry mounting prints especially if they are to be sold because if the purchaser wishes to frame differently or to a different size the mounting board does not have to be cut.