Did you try the Font.CalculateTextWidth function in MBS FileMaker Plugin, version 6.5pr4?
As you can try the example, we can measure the width of a text. This allows conditional formatting in FileMaker. You can for example use a more narrow text font or smaller font size if the text doesn't fit in a field with normal font size. You can even use custom functions to automatically reduce font size or abbreviate text.
Results on Mac OS X and Windows are slightly different due the differences in the rendering code on both platforms. FileMaker may even use other functions to do the text wrapping. But our function should give a good estimate to decide whether something fits.