Why not use Length ( FieldName) and TextSize ( FieldName ; fontSize ) function.
here you can use if-else or case.
I hope it will help for you.
I think you want to have property to reduce or enlarge font size according to the field dimension. Filemaker does not provide this feature but you can use some filemaker functions which may help you for acheiving this.
Filemaker has function called FieldBounds ( fileName ; layoutName ; fieldName ) which can give you the height and width of the field on a particular layout.
Another function is TextSize ( text ; fontSize ) which you can use to increase or decrease the fontsize of the field.
You can just add the script which will calculate the appropriate font size based on field's dimension on onobjectsave script trigger to that field.
Hope this might help you.
One more thing Text formatting options will be lost if the data type that is returned is something other than text or number, no date type..