Image quality and compression is a hard subject indeed. It all depends on how beautiful you want to show it, how beautiful the viewer wants it, The download size of the images, the dowload speed of the viewer etc
Things you should thinks of:
-Am I going to save it as 1 jpg file or tiled. Tiled allows progressive loading. The tiles the viewer sees is loaded before the tiles in the back. But I think this is at cost of performance (constant loading vs 1 time load)
-How fast is the avg viewer downloadspeed of the viewer?
-Do you have unlimited bandwith to your desposal or does your webserver allows only a few GB. Or you within your limit?
-Artifacts you see in photoshop, don't have to be visible in the viewer because of the fov and quality filter (sharpen)
-Artifacts you see in photoshop because you are pro, dont have to be visible for the viewer
-How many colors does your file have? Usually vivid images with lots of colors, are bigger then monotome images.
-Zooming in is a nice feature. And with Feature I mean, it's a nice bonus to have. NOT a MUST! Its great for areal panorama's, but for normal street panorama's where the viewer can see everything easy, its not necessary. Using maxpixelzoom you dont go zoooming in to a pixel level. Which is ridicilous, who wants to see a 100x100 pixel car or statue. People don't care about that. Try limiting your zoom, so you can limit your number of levels a panorama has. This saves in size tremendously.
Here is some numbers from my game:
http://www.darkgfx.com/spel/index.html
Piramide location: 6000x3000. 580Kb for all the tiles together (mostly because its all brownish)
Oasis: 6000x3000: only 200kb! (because it lacks color)
Bazar: 6000x3000: 1.4mb
They are all like 10% of the size you use, but I think they still look great.
With high detailed areal panorama's, I aim for 5-8mb per photo (including all levels). With a normal panorama, 2-3mb is max. I always use like 6000x3000 resolution, sometimes 3000x1500 (because I use 3d, and that saves me render time).
You always have to have your goal (or viewers goal) in your head. What do I want to do with this panorama. Show my skills as a photographer? Or show the viewer the event thats happening (like a motorcross show or a concert). Remember it only has to look good at fov 90 ish, not all the way zoomed in. For example, you make a panorama of a fire that happened in your neighbourhood. People just wanna see the fire, not the pixel of debris falling down in highsharp gigapixel quality. Most users on krpano tend to focus too much on quality instead of the message that needs to be told ;p