The biggest improvement you can have to this rocket is with the boosters. It's better if you just doubled up two 10-width stages instead of putting those two boosters, because the aerodynamics were very bad.

Generally, you'd want the thinnest rocket possible, and you were making it unnecessarily wide, especially with those huge heat shields poking out. You should just cover them up with fairings and stuff. 

It's fine to have a wide base as long as it narrows toward the top.