Supported. It's almost impossible to make 90px look good.
Yes, but people complained about slow loading pages and data traffic on their phones.
You can turn forum sigs off in your profile settings. They have no reason to complain, they're being lazy.
500 being maximum height like before isn't a bad idea.
Anything over 150 would be way too much.
Mind you most people don't use the whole 728, but here's an example.
728 x 500, this after every post wouldn't work.

728 x 150, this could do.
