Take the measurements right off this photo.
Right on your computer monitor, measure a "known" like the diameter of the wheel. We know it's a 14" wheel. If you take a piece of paper and put it up on your monitor marking off what the diameter of the wheel is you now have a standard to go by. You then put a piece of paper on the screen across from the front of the wheel well to the rear of the wheel well and using your standard measure the distance from the line up to the arch. I find that it is almost exactly half as far as the diameter of the wheel making that measurement you are after to be 7". Yes this is kinda crude and it may be off a bit but what does it matter if you are doing a customer mod anyway?
What looks good on your truck is what really matters. Taking this rough measurement and mocking it up somehow on your truck you can get a good idea of how it will look. Mock it up with cardboard or something, be sure that the cardboard is the same color as the truck so it doesn't give you some illusion and get it out side so you can stand back and look at it.