The frames are measured on the inside of the frame. For example a picture listed as an 8 by 10 would be 8 by 10 inside the frame. The frame itself adds about 1 and 1/4 inches to each side. For a total size of 10 1/2 by 12 1/2 on the outside dimensions.
I get a lot of requests on how to make my art. Lol. I can not give away all my secrets.
Mostly I make the frames myself. When I get very busy I hire someone to help out and build some for me.