I think you might've been closer to a solution when you asked how architect's produced 1/2 size sets. Obviously we don't create separate views of every plan at different scales, so we use page setups instead.
We set up our sheets (30x42) full size, and create a page setup that will print them as such. Then we create a second page setup, with a different sheet size selected (say 15x21), and set the scale to be fixed at 50%.
This works perfectly most of the time, as long as your OK with the print not taking up the entire sheet on your letter size paper (since E1 and letter have different length/width ratios). If you can't get past that, or want to have separate and distinct borders for each sheet size, you'll have to make separate views at different scales.
Now, the catch with this approach is that the annotation will not stay at the same size, it will scale in proportion to the sheet. That may be OK, though, because if it didn't scale down with the sheet, you'd probably end up with a lot of overlapping text and lines.
Good luck!
Ross Kirby
Principal
Dynamik Design
www.dynamikdesign.com