I've switched to a larger monitor but now I have a black bar on either side of the screen which is where the old monitor came to. Is there any way to fix this so that the screen fits the desktop? Without any black bars?

Go to desktop

Right-click, choose properties

Choose settings tab and select required screen resolution

benn11 is correct.

You could also try the adjustment controls on the monitor itself.

I've switched to a larger monitor but now i have a black bar on either side of the screen which is where the old monitor came to. Is there any way to fix this so that the screen fits the desktop? without any black bars?
Probably a matter of referring to your user manual to see how to adjust the picture size - most monitors have a combination of buttons or knobs which will allow you to adjust that ; sometimes however it's a little obscure. If you don't have a manual with the monitor, see if the manufacturer has a website and you may be able to download a PDF version of the manual.