You might want to consider what OS the majority of people who will watch your file have.
Playing .mov files on a PC means you need to install the free version of QT or VLC player to watch it. If that doesn't seem like an issue then H264 is probably your best bet.
I would start with the H264 Compressor default.
Set the audio to AAC 128kbits
Set the video to 5000 and then check the summary tab for the estimated size of your file.
Adjusting the data rate will give you an idea how data rate effects file size.
You might want to consider making a 1 min test file of a complex area so you can see how it will look without rendering the whole thing.
Another thing to consider.
mp4 files will play on any PC without QT, VLC player etc. installed (and a mac too) and they should look as good as the H264.mov.
When you mention "the H264 Compressor default," I find H264 listed two different ways as an option in the Settings tab:
• Apple > Disc Burning > H.264 for Blu-ray
• Apple > Podcasting > H.264 for Video Podcasting
Should I drag one of those two up into the "Drag Settings and Destinations Here" window and then tweak the settings as noted in your message?
You can change any setting to whatever you want.
Use the Podcasting to start.
The geometry and encoder tabs are what you want to check.
If you aren't changing the size of the movie make sure the geometry tab/frame size is set to 100% of source.
Under encoder/video/settings start by matching this:
set your audio to this:
The adjusting of the video data rate (presuming you don't change your frame size or frame rate) is what will drive your file size.
The Podcasting in the current Compressor version presets is designed around Apple Device delivery and they are MP4 without very much bit rate latitude.
Darby is showing dialogue windows for QuickTime, which seem to me to be a reasonable choice for you.
Whatever you see for quality on your computer is pretty much what your audience will see when they download.
5 Mbps bit rate for a 720P movie is certainly a pretty reasonable place to start. See how it looks. Bump it up, or down in size and see whether you see any detectable quality difference.