I ran into this problem as well. Can't speak for Northern Tool, but Six Robblees gives three dimensions, width, circumference of the fender (not of the tire it's meant to fit), and the distance between the front and back across the bottom. I'm pretty sure this is the fender I went with: http://www.sixrobblees.com/products/det ... 391&n=f89u
It measures 11" wide, 48" over the top, 33" from front edge to back edge. Here's a mockup with a roughly 32" tire (235 85R16)
Here's what it looked like in an early mockup:
I'm still working out the clearances and suspension travel before I set the final gap. I've got a long ways to go before I need to worry about that.