It is certainly true that 100% is just a benchmark and it is not even
necessarily possible.
However, Word can and should know how big your screen is and what resolution
you are running at, but that isn't really important. Word asks Windows for
the information and Windows tells it whatever you, the user, have told
Windows.
Right click the desktop and choose properties and the font size setting is
hiding somewhere in there. The rather misleadingly named dots per inch can
be adjusted to more-or-less what you want in Vista (XP is a bit more limited
if I recall) and the figure you give it will be the number of dots (pixels)
used to display a 'real' inch. There is just one setting so you will get
distortion if your resolution is not in the same ratio as your physical
screen (e.g. 1024 x 768 on a 5:4 ratio screen will be slightly vertically
stretched)
On a default setting of 96 dots per inch, 96 pixels will be used per inch.
Running at this setting with a resolution of 1024 x 768 means that the full
width of the screen will be occupied by what Word thinks is (1024/96) = 10
2/3". On a 4:3 ratio monitor this equates to a 13 1/3" diagonal screen. So,
if you are running at this resolution on a 17" monitor, say, what Word
thinks of as 100% is actually around 130%.
Probably more than you wanted to know
