If you have the part number for the belt you have, it usually is a code for length and number of ribs (depending on who makes the belt). If you can figure out approximently how much shorter of a belt you need, you can use the numbers to get you in the ball park.
Using Goodyear gatorback for this (good belts)
I looked up a dodge 6.7 and the belt for it is part number 4081261. Ignore the 40, but 8 is the number of ribs, 126.1 is the length in inches.
Using this
http://www.goodyearep.com/ProductsDetail.aspx?id=3128 you should be able to cross refrence a belt that will work for you. Might be a little trial and error. Another thing you could do is use a piece of string, route it over all you're accessories and measure the length. Keep in mind you'll want to make it a bit shorter so that your tensioner can take up the slack. You want it so when all the tension is off the tensioner, the belt should just slip on.