Saving Window Size

I use Yahoo Messenger a lot, and for a while, when I opened an IM window, it would take up the full screen, almost maximized but not quite. I adjusted the window size once, and then each time I opened a new IM window it would stay at the smaller size I had created. For some reason, recently it has gone back to the almost full-sized window, and no matter how many times I adjust it, it will not keep this new size as the default. Is there a way in Windows 7 Ultimate to set a standard size for a newly created window, no matter what program generates it? Or should I be searching in Yahoo Messenger's options for this? I didn't do that before, so thought it must be a Windows feature.

I am talking about the Instant Message window, not the actual Contacts window. The contacts window is a small narrow window, but when I right- click on a contact and click Instant Message, an almost fully maximized window comes up. I want to resize the message window and make it always come up the same size.

Not familiar with this particular piece of software, but apps that always want to open to some preconfigured window size/placement often store that info in the registry. If you're comfortable working with the registry and know how to recover from registry corruption you can search for the entries and change them to see how it affects window size/placement.