Mastric

ah yes

∞ x 140 should cover it. But that's only the page header, when looking at the All Clubs page a different height/width is used...

That would depend on the resolution of your screen or the fixed width of your forum template.
On the default skin with no side bars the image is 1309x140.
On the default skin with no side bars, set to a fluid width of 95% in Google Chrome on a 1920x1200 screen, with the page maximized the image is 1776x140.