Whenever I teach Revit or do a presentation I will adjust the size of the temporary Dimensions so that can be viewed clearly by the audiance or class on the overhead projector.
In Revit 2011 you use to have to manipulate the revit.ini file, a daunting task if your not comfortable working under the hood (bonnet) of Revit.
In Revit 2012 it's a lot easier.
Under the Options button on the Application menue, on the Graphics Tab at the bottom you'll find where you can edit the size of the Temporary Dimensions....a lot easier than changing the revit.ini file!!!
If you need information on how to change the Temporary Dimensions on 2011 let me know and I'll post instructions.