Display size problem

Sorry. Not quite sure which section to put the question in.

I've been using Windows 10 for about a month, having upgraded from Windows 7, and suddenly the display of all items and windows has gone 'very large'. App windows fill the screen, Firefox window is only 75% or less veiwable because the rest is off the screen, and text therefore is very large. Apparently the screen resolution is correct, and I've managed to get the desktop icons down to a normal size by altering them to small icons, but there appears to be no way of altering the size of any windows opened on the screen.
Does anybody have any ideas, because I'm stumped?
Thanks.

Thanks, but that's not it. I think I've found a way around it at the moment by changing the screen resolution from the 1024 X 768 recommended to 1280 X 1024 which shouldn't really work, but has got the windows vaguely back to what they were yesterday (not quite right, but definitely viewable compared to a couple of hours ago).

I'm just wondering if you have already installed the graphics drivers?