OneHost: The chat windows are resizeable (just drag to resize in the upper left corner). You can also pop the chats out into a separate window if you want them even bigger.
You can get an API Key at
www.envolve.com. Just click the big green "Try it for free". It should be pretty straightforward. The "Settings" tab is in your vBulletin Admin Control Panel. It should be the top button on the sidebar on the left. The "Options" button will appear directly below that when you click on Settings. Here's what it looks like:
Hope that helps!