Mechanical engineer Perry Pattiz suggests dimensioning in AutoCAD’s paper space, and provides a tip that will make it easier to do so.
"A lot of people are still dimensioning in model space, which requires changing the size of the font every time you change the scale at which the drawing is being plotted. Instead, do all of the dimensioning in paper space on a Layout view, and the text will be the correct size independent of the plot scale. The trick to having the correct dimensions is to set the variable DIMASSOC to 2, then making sure to select object snaps on the model in the Layout view."
Notes from Cadalyst Tip Reviewer Brian Benton: This is a debate that seems as if it will never be resolved — and that’s OK. Where should you dimension? I believe that either method will work, as long as specific steps are taken. This tip suggests that you dimension in the Layout views (paper space). If you do, then please set DIMASSOC to 2. This setting will allow you to dimension with only one dimension style. You won’t have to create styles for different scale factors. The dimension objects will follow the objects they are dimensioning, meaning that if your line moves in model space, then your dimension in paper space will move with it. Just make sure you snap to that line.