OK, I was curious because there's more than one discussion that could be had on the subject of prism size. I think the main way prism vignetting happens in spotting scopes involves a certain focusing system that is mostly used in inexpensive scopes now, but was very common in the past. It works by moving one prism in a Porro prism cluster back and forth to change the physical length of the light path between the objective lens and the eyepiece. The problem is that there is only one optimal position for that prism and as it is moved to lengthen the light path it introduces increasing vignetting of the clear aperture as the focus distance shortens. Typically a scope using that system with an 80mm aperture at infinity focus will be stopped down to something like 60mm-65mm at 5 meters. I'm pretty sure the Celestron regal uses that focuser and probably some inexpensive Opticrons as well.
I've seen examples of careless designs causing a loss of clear aperture. The Zeiss Gavia I tested, for instance, had an 85 mm objective lens that was inexplicably stopped down to 80mm by an undersized internal aperture and there is the infamous example of the 95mm Zeiss Harpia intentionally designed to lose aperture at low magnifications resulting in a 57mm clear aperture at 23x.
Some possible effects on image quality from large prism size unrelated to vignetting would include the use of oversized Schmidt prisms in most Nikon Fieldscopes that act to prevent the roof prism's edge from splitting the objective lens's light cone and the extra internal glass length of large prisms adding spherical overcorrection that may help reduce the typical under correction of most short focal length spotting scopes.
