Revit 2013 changes the means to reduce the overall size of the text displayed in a dimension string, particularly noticeable to people using imperial units. It was tied to the project units and you had to alter a dimension style to not use the project units settings in order to use it.
This is the new location for the option, found in the Type Properties for a dimension style.
This means your dimension styles can continue to depend on the Project Units as well as suppress the spaces in dimension values. A subtle but more logical approach I think.
However in testing it appears to be broken... The option for Suppress Spaces exists in the the Project Units dialog. If I turn on the feature it affects dimension styles downstream and the option in the dimension style should then work as an option to remove it but it doesn't do anything. Odd...