Usually, when I'm on a site and I click on a link the window opens up full size, but sometimes, for no reason at all the windows open very small. How can I stop this, what makes it happen?
In your current browser click on the button at the top right (next to the red one with the cross) which will "Restore" it to a default size.
Any new windows are generally set to open at this size so if you "Restore" and then drag the corners of the window to make it almost the same size as a maximised window then your problem will almost disappear.
After you're done with all this you could maximise your current window again