What's the aperture/sensor size for the mid-2011 27" iMac FaceTime HD camera?
tl;dr - As a starting off point, eyeballing it I'd estimate around ƒ/2.2 - ƒ/2.8 aperture.

This isn't an exact answer, I put the iPhone 7+ front camera (which has a ƒ/2.2 aperture and a 'focal length in 35mm' of 32mm) next to the 720p FaceTime HD camera on a macbook. Eyeballing it, the image on the mac seems to be the same or maybe ~5% smaller in FOV. So I estimate around ƒ/2.2 - ƒ/2.8 , maybe ƒ/2.4 in aperture.