Most likely a 23" (58cm) frame. You might want to compare this size against a larger 25" frame to see which you like better. The Grand Prix was a lower-end bike with a 20-30 high carbon frame, as opposed to the Reynolds 531 found on a Super Course.

Probably a fair price if it is in good shape, it fits you well, and you like it.

Useful Raleigh Grand Prix link ...

http://www.sheldonbrown.com/retroral...rand-prix.html

I could be wrong, but this looks like an older '70s model upgraded with Suntour components and a cotterless alloy crank set ('77 style). Upgraded to alloy rims?