I have an iphone 4 and jailbroke it the other day...everything was fine. I'm new to jail breaking so I was installing some background apps and themes. Then after one of the restarts, it loaded the main screen fine, but no matter what app I launched, it would load full screen then shrink to a very tiny window in the top left of the screen. (See Pic)
The Rock & Cydia app worked but were unusable as far as trying to hit manage and remove the apps.
Everything seems to be working again after a full restore. The jailbrake went smoother this time. And the apps load like they should. I seen there is an option to reinstall jailbrake without restoring, but you have to use cydia, and it was so small your fingers couldn't hit the right fields, buttons.
You must have SBsetting innstalled from cydia. or another apps from cydia which you can go into safe mode.
Since sb setting doesn't go small you can manage to open this.
Press "Power" on sbsetting choos "safe mode"
Open an apps in safe mode. Telephone-sms or somtings else.
Swipe finger over status bar and "restart" iphone.