I don't know if you have a different version or something but I use Edit -> Change Image Resolution.
Lowering the resolution will of course make everything more blurry. Maybe if it is too blurry you are lowering the resolution too much?
If you are lowering the resolution by the minimum amount possible to fit within the site's requirements and it is still blurry then maybe you need to break up your pages into multiple images? But that shouldn't happen since the image dimension guidelines are meant to be able to give reasonably high quality images.
I'm not sure about resizing comic boxes since I don't use that functionality. I just draw panels manually on a separate layer since it seems easier.
Hope some of that helped somehow 