I like to find frames with nice mats. otherwise, the stuff gets smooshed! The 6x6 joy frame in my gallery has ribbon on the side 'cause it was too thick, and the glass cracked a bit.
I like the look of 8x10 with a 5x7 opening. The silver ones in my gallery are really super deep, and the words Adri wanted to do were a little longer than I would have chosen, so we just went over the opening!
Oh - and to answer your question, I take the frame first and then work with what size I have.