As for forum layout, I tend to trust widths up to 600px~ to be generally safe, though text layout is complicated to scale well - access the forums via mobile for maximum carnage.
I've been fiddling with the idea of positioning images via float left and right for start/end of posts, respectively, and trusting myBB encoding to format text properly... but yeah, after looking at my posts on other computers this kind of info was something I started to wonder as well.
