The dimensions print the same size (on paper) regardless of scale unless you are rescaling the drawing on the layout. To setup drawings at different scales make two different views from the same source.
Since the smaller scale drawing cannot contain as much detail the different layers are necessary to separate the notes and dimensions not shared by the two drawings. If there are notes and dimensions which are to be shared, you will typically need a separate layer for these as well. That is unless the smaller scale notes and dimensions are a true subset of those on the large scale drawing - then they both share the smaller scale common stuff and the additional detail is turned on for the enlarged plan.
In general (as you probably know) it is a dangerous practice to duplicate information on different drawings. It is not as problematic as in the old manual days since the information is associative with the same source, but it is still extra work and can lead to subtler problems.