Can't Resize Window

I use an external FW drive for my OS to drive my iBook and my G5. Mostly the G5. That's away under repair at the moment so I'm using the iBook.

What is happening: windows are sized as if I'm still on the G5 i.e. very big for the small iBook screen. And when a window goes below the bottom of my iBook, I can't get to the size box to resize it.

How do I resize such a window?

Thanks in advance for any suggestions