So, I'm going to be blunt here, because I like open and honest communication. If someone that actively disliked you looked at your bow and could argue with a marshal that it's no longer a bow, that's a shield; Or if using it on the field would make somebody angry that you've made a bow that wide, you're probably doing it wrong. The goal is never to find a way around the spirit of the rules to gain some kind of advantage. So let's be honest here, why do you ask how wide a bow can be? Is there some kind of specific bow you want to make? Do you want to make a really cool prop design in mind and your design has a wide point which you're trying to be fair about? Do you want to make a really wide bow because it can block things a lot better so you don't get hit as much?
If it's 1 or 2, talk with your local marshal, give them design specs, sketches, mock-ups, anything along those lines, and they'll work with you to either say it's good or not. If it's the latter, I would seriously recommend going with a standard bow construction. It's not worth the hassle, time, or arguments such a design might bring up.
To use a somewhat similar situation. A player came to me as a marshal to see if they could design a trap to do a "really cool thing". It was a novel idea, but outside the scope of the rules. Our chapter's marshals talked about it, and ultimately decided it went against the spirit of what traps can do, and to even be remotely possible, it would be in the realm of ritual magic. Even then we discouraged it, because again, it was outside the scope of the rules. I praised the player for their creativity, and keep in touch with him to this day when plot needs an interesting prop for a plot.
Develop that working relationship with your marshals and plot team. It's the best thing that you can do, ever. We're all here to have fun, where we can trust each other to play to the spirit of the rules.