(http://www.papaspizza.net/ What's with the 30 $ pizza's
It's been a while since I've been there, so it's hard for me to say for sure (although cost of living in Corvallis is a little higher than most of the area around it), but I can take a guess...
You'll notice that the sizes are given in terms of slices, not diameter. If I remember correctly (and it *has* been years), the slices are pretty consistant in width.
Last time I ate there (last year), I got a medium taco, which was split between myself, my wife and an old college buddy of mine. We still have leftovers... and that was with me stuffing my face, since it'd been many years since I'd had a taco pizza. I'll hazard a guess that the medium was in the 14-16 inch diameter range.
If my assertion that slice width is the same, then the large is in the 19-21 inch range.
I don't normally get my pizzas that large, so I have no idea how that stacks up with "normal" prices.