Is it possible that the active layer is below that on which the thing you are trying to dimension lays? I was working on something the other day and started to insert a dimension which wasn't showing up. It turned out I had made a layer lower in the stack active. the dimension was indeed being made but it was hidden behind the SU viewport on the layer above.

Thanks for your reply - I'm using the same 15" MBP retina with "Best for retina" resolution. Window box text is fine, I guess I'll have to get some glasses!