Math Problem

I have prepared this recipe (the "espresso pecan brownies") according to instructions. Instead of a 13"x9" or 8"x8" pan, however, I am using a pan that's somewhere between the two -- I can't measure it right now because it's in the oven -- say, 11"x8" or so. My question is this: how long should I leave it in the oven? The only other variation from the text I can report is that I'm using walnuts, not pecans.