I've run plastic spinners on 4-strokes for years with no problems. In fact, if you over lean it, and the engine kicks back, the whole prop+nut+washer+spinner comes flying off as a unit, making it easier to find, but more of a hassle to put back on .

The reason I heard to not have the spinner touching is that the blades to flex a little in the air. If the prop is touching the spinner, it's slowly rubbing, and that means cutting in to the prop. With a soft plastic spinner and a harder prop like an APC, I don't know if it would matter as much, but it's still a good idea to have some clearance. Also, if it's touching, it's possible you are warping the spinner as you tighten it down, which will cause a vibration when it's running.

As for spinner size, the inner few inches of the prop really don't do much anyway, so don't sweat covering them up. If anything, a larger spinner will often make a plane go faster as it smooths out more of the airflow around the fuse. I'm using a 2.5" spinner on a 9" prop, and the plane runs like a scalded cat.