Short answer, YPP stores the client window resolution in a java preferences setting.
I have often wished that the YPP client supported window resizing. It doesn't, and I accept that.
Given this little bit, you can go tweak your system and get any client size you want.
For windows users, those preferences are stored at HKEY_CURRENT_USER\Software\JavaSoft\Pref
The values you want to change are: display_height and display_width
I'm currently running it at 1200x700.
I wanted it to be almost full screen, but give me a bit of real-estate on both sides of the screen for IM tools and the such.