there can be a few ways to reset text size.

#1
go up to the tool bar and click View
slide down to Zoom and then slide to Reset.


#2
Or you can press CTRL - { the minus sign button}
each press of the minus button should make the text smaller
the plus sign makes it bigger


Or if you were in the display settings of your computers control panel and made text larger there then we can work thru that also.
but try the View or the ctrl - first

Or if you changed sizes in your browser settings...we can work on that if it is the case.