In the previous beta/build, Windows XP/OS X would change the resolution automatically when going full screen. In this new beta/build, I just get a low resolution in full screen mode. The "Change Mac OS X resolution in full screen" under Preferences -> Appearence is ticked. I've tried unticking, restartng Parallels, ticking it again to no avail. Likewise I've tried restarting the host. I'm running 10.4.8 on a Macbook 2.0Ghz Core 2 Duo. M.
You don't want to check "Change Mac OS X resolution in full screen", you want to check "Change guest resolution". (At least that's how it worked in 1970 and I presume it hasn't changed in the 2.5 betas.)
I don't see that option under Preferences -> Appearance, only the "Change Mac OSX resolution in full screen. This is in build 3106.
Either way (I've not made any changes to the configuration), the solution to this problem is to uninstall Parallels Tools for Windows inside the Guest OS, reboot, and re-install them. I'm now gettng full resolution when going full screen. Looks like a bug with the Tools upgrade process when installing a new version of Parallels.. M.