From a post by Scott Latch, the new Technical Product Manager for Revit Architecture, at AUGI's Revit community forums.
Open Revit.ini file in Notepad.
The file is typically located in C:\Program Files\Revit Architecture 2008\Program.
Add the following line to Revit.ini in [Graphics] section:
Where N is a number larger than 8 (default hardcoded value). Good starting point is twice the default size (i.e. 16-17).
If the size of temporary dimensions is still small, repeat from step 1 with larger number.
Please note, the [Graphics] section may not exist. If this is the case add a line that contains [Graphics].