When working in 3D in Revit, you *can* use the normal dimension tool *if* you first set the Workplane. However, this can get annoying. Why not make a 2 point adaptive family, with a reporting parameter, so that you can just place the family (two clicks) and then check the instance parameter as shown below?
Update: V2 with arrowhead and tag in project Download here
Adaptive dimension family (2016), Download here
Revit wants you to make your own better, faster way sometimes…