ricknkim,
no, the spinner you have is fine. Do as your LHS recommended and trim the spinner to fit. If it won't fit on the prop shaft then drill the hole out to fit. If it won't fit over the prop then use your dremel to enlarge the cutouts to fit over your prop, just try to remove the same amount of material from both cutouts in order to keep the spinner balanced.

I've never built the CG Chipmunk, but I've built other kits with a cowl on them. Usually the cowl is formed with a specific spinner to match it. If you go with a bigger spinner it's going to break the lines of your cowl/spinner fit.

Hope this helps