Everything seems to have gone larger on my screen ?

Check that you have not changed your resolution by accident :-). The higher the number the smaller your text. This is sometimes adjusted when launching full screen applications that require a lower resolution. (in Vista and Win7 you can click the start button and search for Resolution then select 'Adjust screen resolution').

If its in an internet browser you may have accident changed your magnification, in this case press and hold Ctrl and scroll with your mouse wheel until you hit your desired screen magnification :-).

Is this on web-pages?

If so, on the bottom right hand corner of any web-page there should be a little icon of a magnified glass with an percentage number e.g 100%, 80%....this is the zoom which the screen is on. You need to click on that and change the zoom percentage.

right click on your screen and select properties???????