PhantasyArtStudio, really wonderful questions, I am personally using unstretched canvas for the main reason that:
-I live in England right now and need to ship my work internationally (rolling is the most affordable and safest option) - I also provide numerous presentation suggestions so that my clients can choose to frame, stretch, hang as is (see any original painting listing of mine for an example)
I imagine that 8x10 would probably sell faster as you could put a smaller price tag on it, but I personally like painting bigger so I do mostly around the 18x24 range (varies slightly with European sizing). I just try to keep my prices reasonable to encourage more sales ;o).
I hope this helps. I think if you go the stretched or panel route you'll want to figure out packaging/shipping early on and which countries you can afford to send it to without breaking the bank ;o).
hugs,
brienna