Shoutbox

You mean window as in: Conversation Window background, or the Window with your Display Picture?

The size of the conversation window varies from user to user (and computer to computer), so you will just need to make the correct stretches to it. You could find the approx. size by taking a print screen of your conversation window, and finding the w and the d of your window, which then you can use an editing program to crop + resize the picture at your discretion.

Your Display Picture has a fixed sized of 96 x 96. Therefore, you could simply use an editing program and resize + crop the picture at your discretion.