You can breakedown the project if there are gains in terms of proudcutivity and maintainability. The no. of developers working on the project and the work breakdown should factor in this decision.

Having said that, There are no threshholds that I know of which you might be near crossing. To reduce likelihood of source code corruption in a big project like this esp. vcx's in addition cutting down the size of the exe, try recompile all option every week or so. Also unchecking debug info check box should save you like I stated before about .5MB in average.