While in PFK hold down the control key and scroll up with the scroll wheel on your mouse. This may be what you did, by accident, scrolling down rather than up, to make it small.
Oh wait. That controls font size and images. Is that what you mean?
Otherwise, look up in the upper right corner, you will see a line (-) , a little square and an x. Click on the little square to see if that enlarges the screen space.