Contest framing question.

[ggt1_02]

I have entered a contest where the gallery supplies the frame and you are allowed the fill it with any medium you choose. The question I pose is what size print should I use to fill the frame? Printing and framing anything but 8x10's is new to me. I would like to mat the print also. Here is one of the shots I am thinking of using. Any help would be greatly appreciated.

http://farm3.static.flickr.com/2237/2419363787_0aeac94514_o.jpg