Somewhere in the 10-to-16 inch range the atmosphere limits additional resolution gain, and on many nights even 8 inches is max for resolution. There are nights when 10 or 12 inches clearly outperforms 8, but 8 is just barely big enough to see Jupiter's moons as disks on the best nights, and on nights of average seeing you won't get much more out of a 10-inch scope, especially since you'll have more trouble getting good color correction in a manageable focal length with the larger glass. I don't buy the "you'll like the image better on bad nights with a smaller scope" line. You can always stop down the aperture to imitate a smaller scope, but I never actually do so.
Mr. Kidd makes a valid point about aperture and planets, at least partly: you don't need more than 8 inches for light grasp, since Jupiter and Saturn are plenty bright. Gains in resolution go up only linearly with aperture, and often atmosphere limits the resolution. Light grasp, which you don't need for planets, goes up with the square of aperture and is less dependent on the atmosphere. So for Dobs, aperture rules.
The best images ever made of Jupiter by any mortal at visual wavelengths are by Antonio Cidadao (hope I've spelled it right) with an 11-inch Schmidt-Cassegrain. Sky & Telescope had an article featuring his work. No professional scope of any size can beat him at visual wavelengths, save the Hubble. So 11 inches or less is enough, and the gain from an extra inch or two of lens is probably outweighed by much greater expense and worse color correction. So go with 8 inches, I say.