Computer help required

[joebloggs]

I've some how managed to increase the size of my desk top so that I can barely see the lower icons (speakers, internet connection etc) and web pages are also to wide etc

I've looked at screen size etc and the computer says I have the right settings for my monitor, so why can I not see it all

Do I need to use the F keys and alt and or control or something similar.

Thanks in advance