FHHubber is right on, it's totally an appearance thing, the guy at the shop is clueless.

Next time you talk to him, ask how 4-strokes and other engines with rear-mounted carbs work .

(for intersting carb setups, those of you who heard about the Trans-Atlantic model thing might find it amusing to know that the engine used (an OS .61 FS, modified quite a bit), had the carb mounted on the end of a rather long bit of plastic tubing, and the carb was actually back inside the fuse, behind the firewall. It was also a carb from a .10 size engine, itty bitty.)

I've used spinners bigger than the carb many times, no big deal.

One thing a bigger spinner can do, is make your plane go faster. If you have a stick-type or other model with an exposed flat firewal, mounting a spinner that is about the size of the fuse, or a little bigger, will help reduce the drag of the front of the plane, and make you go a little faster.

Oh, and if you are worried about covering too much of the prop, don't be. The vast majority of the thrust is generated by the outter portion of the prop. You can cover quite a bit with out much of an effect.