I have seen this question before, so I did a search for "Spinners" and came up with this;

Optimum range of spinner size is 20% - 25% of the prop diameter the advantages of using a spinner , the spinner eliminates the drag on the engine from the propeller root-transition zone, where little lift, but lots of drag is produced. The engine will actually rev a bit higher & make a bit more power when equipped with a properly sized spinner .

There are a couple of rules:

The prop blades shouldn't touch the sides of the spinner , or the rubbing/vibration could make either, or both, the spinner & prop fail.

A spinner shouldn't be larger than ~ 25% of the diameter of the prop blades, or you will start to lose prop efficiency -- eg. a 3" spinner is about as big as you want to put on a 12" prop. That isn't cast in stone, but it is a reasonable max size relationship.

Have a look at these links for extra information.

http://www.rcuniverse.com/forum/m_34...tm.htm#3497546

http://www.rcuniverse.com/forum/m_39...tm.htm#3923950

http://www.rcuniverse.com/forum/m_25...tm.htm#2583467