Shoutbox

The size and thus also the proportions depends on:
- The size and state of the window of Windows Live Messenger (but in your case this is maximized)
- Which version of Windows Live Messenger you're using...
- If Windows taskbar is visible or not...
- If Windows taskbar is locked or not...
- The size of Windows taskbar (and maybe other sidebars on the desktop)...
- If you're using a scrollbar or not in the contactlist!
- Which menu's you're showing (eg: topmenu with window border, etc)!
- Which patches you've applied!

So you see, it depends on quite a few things. Without knowing all these factors there is no way of giving you any specific size (or proportions which are directly linked to that).

eg: paperless' example is totally wrong in my case on a 1024x768 monitor; not to mention the light blue border is also in his screenshot, which isn't part of the actual background actually.

So in the end, you can better measure this for yourself. Thus if you always use the same configuration you could take a screenshot of the maximized window and paste it into a graphic program. From there you can measure the inner size (only the white area, excluding the scrollbar) of the contactlist in pixels.