Did you change the zoom?
On the bottom left of the window, there is a toggle to zoom in and out.
Yes, I have tried Zoom. Zoom is not a factor. The standard or default size is off. Zoom does not take it to the proper sizing.
Go to Edit > Preferences
On the General tab, there is an option for 'Enlarge window contents to improve readability'
Give that a try. It may be that you have been using it checked, however something caused it to become disabled.
I have tried that option. It magnifies the scree approximately 150% but it is not the same screen size as normal (slightly larger). Another observation was made during this 'corruption'. When entering text in a field, words at the end of the line are chopped erratically during wrapping to the next line. This has never happened before. It appears there is a problem with the filemaker program since this does not occur when utilizing the same file with sharing computers and it happens on multiple files. Furthermore, the resolution problem occurs with a trial versionof Filemaker 11 and Filemaker 8.5 has been re-installed.