Ok, I found a Michael Sweeney video on UTube that explains how to do this...

Example:
50 Pixles per 5'
How many feet across (Feet x 50 pixles)
85 (4250 pixles) by 130 (6500 pixles)
Save As
Rectangular section JPEG
Options Width = 4250, Height (set to ungodly high 30000 is good)
(BE SURE CROP IS ON)
Image will crop height automatically

Fairly simple.