Picture mats should be 2 times size of frame( thickness).
Make sure mats are wide enough. A mat that's too skinny won't set off the piece it surrounds. Here's a handy rule of thumb: For the best effect, make the mat at least twice as wide as the frame. You can go wider, but be sure the art doesn't get lost.
wider mat. The piece of artwork becomes large, in charge, and dramatic with just a crisp white frame.