I think the answer is "no" - meaning yes, you are right, that you cannot change the automatic transcode settings to scale to fill.
I think this has not been asked much because there are still many users that do not like Encore doing the downrez form HD to SD.
Make a: Feature Request
I agree with Stan. Do the transcoding from PP's export dialog where you have more control.
I normally HD-to-SD transcode in other apps but this double feature would mean too much trial and error transcoding to get the sizing just right so everything will fit on a disc without too much empty space.
How do you guys normally go about balancing your video transcode settings to maximize disc space? Most of the time with my projects, it is never just a simple video to disc - there can be multiple features, subtitles, DTS pass-though audio tracks, audio commentary tracks, featurettes etc.
It can be very complicated with the extra stuff.
However, since you already transcoded with automatic: just look at the transcoded files and see what Encore picked - Gspot or Mediainfo.
How do you guys normally go about balancing your video transcode settings to maximize disc space?
I actually don't worry much about disk space, I worry about quality. Towards that end, I use external encoders that have the superior CQ mode available. Specifically HC Encoder for DVDs and Xvid4PSP for Blu-rays.
If the quality settings I prefer create a file too large for single layer disks, I just use dual-layers.