What he said.

I apologize. Our current jamb size is standard 4 9/16. Would making the jamb 6 9/16 give us any more projection?

"No the jamb depth does not effect the mounting position. 6 1/2" jambs means it has extension jambs for a 6" wall already installed."

Reread what I typed. The window mounts the same way no matter what the jamb depth is. The 6 1/2" jamb means the jamb extension is already applied and extends to the interior to cover the wall stud and drywall edge.

Alison...while in theory your math is correct in that it would extend the window out further, the reality is that you don't want to push the window out and have the weight be supported by a jamb extension in this case.

+1 to WoW's comments. Changing the jamb depth doesn't change the exterior projection (ie: the portion of the window designed to project to the exterior of the home), however you can tweak the total unit depth/projection by chnaging the jamb as you've alluded to. This is not incredibly uncommon, but discretion must be excersised as once again, extending that out farther is pushing part of the window out past the exterior wall that is not designed to be. Proper mounting is a consideration (although a good chunk of that weight will be supported by cables or legs), as is covering those "exposed" sides with a trim build out or something to that effect.

Lastly, what are you trying to accomlosh by getting the window to extend out farther? If you are trying to meet old soffit or cover a paint line or something like that, keep in mind that the contour/shape of a new unit won't match the old one perfectly even if the full projection measurement is exact. There may be better ways to solve whatever issue you have.

our current bow extends 13 1/2. The new bow will only extend 12 1/2 and we have soffits so there may be a 1 inch gap between soffit amd bow. Without getting new soffits, what are some options for that little gap around the bow where it meets the soffit?

I was thinking that it may be something along those lines. For the reasons that I mentioned above, you aren't going to get this new unit to line up perfectly with that soffit anyway. The two options are 1) replace the soffit, even it just that section, or 2) add some trim to bridge the gap. Adding a trim board to the top of the unit should do the job.