I cant get a full screen ,

SOURCE: screen shrinks

Possibility: Dust accumulated within.
Disconnect the monitor, and open it up. With a dry small paint brush and a vacuum cleaner, remove as much as possible. Especially around the tube and the ultor (the cable connected to the rear of the tube, usually has a rubber cup- like fitting on the tube end), then close up and test again.
Still doing it? This part requires great care, if you don't feel confident about working around a live chassis, DON'T!!!
Disconnect and open the monitor again. Connect up without its cover. Look around the chassis, you will see a number of adjustment potentiometers, possibly with markings such as "width" and "height" etc. Turn on the computer, and with a PLASTIC adjustment tool or TV alignment tool, (you can make one from a plastic knitting needle even), carefully MOVE each of these adjustments a bit, THEN RETURN THEM BACK TO THE POSITION THEY WERE IN.
Dirt often causes these controls to become intermittent in their wiper contact to the track.Moving them can restore a better contact.
There is also available fron electronic component suppliers, a pressure- pack contact cleaner product that can be sprayed onto/ into these controls, and may help if moving them does not.
If the above does not cure the problem, there is probably an electronic fault.
Remember, if you don't feel confident about poking around inside live equipment, DON'T, maybe you have a friend or relative who knows how.!!