Put yourself in the shoes of one of my interns, who was asked to use representations to replicate a 1:100,000 scale geologic map.  In less than two weeks she got it done, and did so well that the next thing I asked her to do was to test ArcGIS Server and make a map service with her map. Those intricate little point symbols on geologic maps that are rotated by their strike angles and labeled with their dip angles were just too small to be seen clearly on screen.  We needed to make them bigger. 

I thought, great, we'll use overrides (using a field in the data to drive the size of the symbols, just like we already were doing to rotate the symbols by their strike angle).  Then it turned out we had to not only add a field (type = double) to our data, but also edit each representation rule (we had more than 100) to use our new field.  

Hindsight being 20/20, I would have had the person who set the rules up in the first place do their job a bit differently. Meaning I now think it a good practice to assume that some override fields should be pre-staged in the data model. This is definitely a good idea if there is a chance that you want to serve your paper map as a map service using ArcGIS server.  Once these fields exist and are used by your representation rules, you only need to copy the feature class and then calculate those fields to change the sizes (or other properties of your symbols) in one step.  

Once we had adjusted our representation rules it was quite easy to very rapidly experiment with several marker sizes until we got the best look for the map service.