If you have successfully jailbroken the beta version of iOS 5 on your iPhone, it is highly likely that you were confronted with the quarter screen issue, in which the full screen display gets shrunk to fit in one quarter of the screen.
However, there is a fix for the quarter screen issue, which is caused by the jailbreak app Activator, according to idownloadblog.com
Click on Power and select Safe Mode
Once booted into safe mode, click on the status bar when it says Safe Mode. This will bring up a prompt; click Restart to restart into normal mode
Launch the offending app and the issue should be resolved
Unfortunately, there seems to be a slight glitch with the above fix. It is only temporary and you will have to follow the same steps every time you reboot!