Increasing size of Revit Structure's temporary dimension's font:

1. Find Revit.ini file in Windows Explorer search:

Example location:
C:\Autodesk\Autodesk_Revit_Structure_2011_English_Win_32-64bit\RevitSetup\RevitStructure



2. Then add the following text:

[Graphics]
TempDimFontSizeInPoints = N

3. default N value=8, good starting point is double that which is 16.

4. Save file

5. Restart Revit Structure