screen size

cannot get my imac back to full screen tried arrows in top right but that does not do the trick. have ab out 4inches on both sides if active screen

my imac is not allowing me to use full screen. the arrows in top right corner does not do the job. everything i view now has a margin of about five inches on each side and limits some of the screen size need to get back to full size screen

Have you tried a reboot? Which iMac? What Version OSX? What application is not full screen? Do you mean that you have black bars on the sides of the display, or that the application won't go to full wide? You haven't given us much to go on here.

Try this:
Go into System Preferences > Displays
Click on the Detect Displays button

Maybe your resolutions were changed and this will restore the default. Not sure, but worth a try.

My iMac is OSX Version 10.7.5
All applications refuse to go full wide with black bars on both sides sometimes impeding information on right.

Does the desktop, with no applications running, show the same black bars? Have you tried a reboot? Which iMac? Have your done what toMACash suggested?