I have a Hobbico superstar with a TT .46 motor and I am looking to install a plastic spinner on it. Right now I have a dome nut, but want to convert it. The problem I am having is my LHS claims the only have car guys available to help me, and they are not sure what size I need. Can anyone help me out here???

Use a 2" or 2 1-2" either or will work plenty good. If you are worried about diameter on the shaft it doesn't matter the spinners come with spacers.

Make sure the prop is NOT touching the edges of the cut out on the spinner! This will help avoid damaged spinners and props. It may require that the spinner be trimmed a bit to allow the prop to clear, do it equally on both sides to keep things in balance. I bit of extra space doesn't seem to be a problem so keep it equal on both sides.

Just get a propeller that fits the profile of the spinner on the cowl. THat is going to be your best rule of thumb.