I recently bought an Acer Aspire One and seem to have exactly the same problem. I have worked out that by pressing CTRL+ or CTRL- I can increase or decrease the size of the font manually but I still have no idea how the changes originally occur.
On many modern laptops the mouse touchpad is capable of zooming in and out by using two fingers on the pad at once. pulling two points away from each other makes it larger and "pinching" it reduces the size. It's similar to an itouch or iphone's ability to enlarge the screen for easier readability. Sometimes on my Dell XPS my arms my rub against the touch pad and change the font of the screen. try putting two fingers on the mouse. keep one still and drag the other away from and closer to the still finger to see the results. good luck.
I have a Acer 751 netbook and kept getting the same problem of big font small font and have just sort out how it is happening actually looking on the net to find a reason whand y it was happening and it seems that if you hold the touch pad to long with one finger and try to move the curser at the same time it rolls the screen size just like what was mentioned above by CTRL+ or CTRL- I if you see a little cross instead of the arrow the and touch the touchpad screen size changes
when you use the touch mouse, make sure you dont use 2 fingers because when you put 2 fingers on the touch mouse you change the size of the screen and font. so use 1 finger at a time... unless you want to change the size.
Hye, did you try to check for a virus? sometimes there are games that changes your screen resolution in order to run the game but they bug and fail to return the resolution to the previous state. under what occasion does these changes occur?