I'm not an expert, but I'll take a stab...
It sounds like you have a specific width in mind from the outer edge of one front tire to the outer edge of the other.
The frame width and control arm lengths will have something to do with it, and depending on what you choose, those are going to be relatively fixed. Plus, as I understand it, you can't just lengthen or shorten the control arms without affecting the roll center of the suspension (someone please correct me if I'm wrong here).
On top of that, you're going to have to have at least a general idea of what width tires you are going to have in there and within that tire width window, your wheel offset will determine the final width.
So, you have the frame rail width + twice the length of the control arms (only the length they protrude outwards of the frame, which, depending on how they're mouted could be less or more than the actual arm length). On top of that, you have a certain amount of width from the ball joint to the hub assembly surface (x 2). Then your wheel offset will determine how far in addition to that the actual tire edge will be (again x 2 for the total track width).
There may be a range of hubs available also that will alter where the wheel bolts onto the whole assembly - depends on what you're using.