While I'm not sure about the material selection (plastic vs. metal for a .75 size), I will say that spinner size is based on the shape of the fuselage of the plane. Choose a size that looks good on that plane.

I have plastic spinner on a .46 2-stroke, and a metal spinner on a .91 4-stroke. Personally, if it was a nice plane, I'd go ahead and get a metal spinner for a .75, but I would imagine you'd be ok with a plastic one too, especially if you want to fly with a spinner by tomorrow.