The options found in Engraving Options > Rests > Multi-bar rests > design give good control over multi-bar rests but they do not affect rests lasting a single bar.
Combining the Do not scale option that Janus shows with a lower value for Single multi-bar rests width (I think it’s called something along those lines, not at my computer right now) give me single bar rests that are a lot shorter than multi-bars. I can make a mockup tomorrow but it’s probably worth playing around with those values again after changing that scaling setting.
The proportions are right but the 16-bar rest is now too wide for my taste. So I shorten it by lowering the value for Maximum width for a long multi-bar rest:
Now the proportions are wrong so next I’d like to decrease the width of the single bar rest a bit. I would assume that lowering the value for Width for a single bar multi-bar rest will, well, decrease the size of the single bar multi-bar rest (I expect that because I’ve read posts on the forum by Richard Lanyon stating that Dorico views a single bar rest as a multi-bar rest that happens to be only one bar long - if that setting means something else then I really don’t understand the distinction between the two settings since even a 128-bar rest spans one bar). But it doesn’t. I can set it to zero and nothing happens. If I put in an extreme value, it’s not the single bar rest that changes but the 16-bar one: