Whoa. I just had the same problem. I went from a large monitor to my laptop, and iTunes' resize handle is too far down. Normally, I can zoom (i.e. maximize) the window via the green plus in the top left of the app, but that only switches to the small iTunes player. NOT helpful.
Here's a free/donationware tool that'll help:
http://mindflow.dk/software/windowwrangler/
Apple should be embarrassed that we have to install a 3rd party tool for this! This is GUI 101, here. At least Windows lets you resize from various corners, or use the Alt-F4 option and then use cursor keys. If my PC friends new about this, they'd never let me live it down. I'm a little ashamed right now...