Changing the Avatar Width in Forums

[Ferretpelt]

I'll admit - I'm just learning CSS.  I've been trying to hardcode the width of the "user information" column in SMF and been having no luck.  The folks that I have using the forum are proud of their avatars, and they tend to be  a little wide.  I can't seem to get the color width to set to a hardcoded 400 pixels vs. the "variable by display.

Can anyone give me a pointer?