To get the pixels from inches, multiply by the DPI - dots per inch. Therefore, 10" at 300dpi (pro mag quality) is 3000 pixels, as mentioned above. Divide to go the other way. So 2100 pixels is either 7" (300dpi) or 14" (150dpi). Also, unless you're rendering for 300dpi, or you're using very detailed volumetrics or cross hatching, or possibly severe DOF - you never need to go above 64 rays/pixel. For the above image, I doubt you'd need to go above 16. I'm doing a piece at the moment on 9 rays/pixel, with massive amount of volumetric cloud, and it's looking fine. In fact, when it's done I'll probably smooth it because it's looking a bit too sharp.