Based on your reply, I deleted and recreated the container and installed RetroPie immediately afterwards. It worked! It's installing now. I'm guessing there was some other package conflict with another app installed. Thank you for your help.
On a PC, RetroPie will need an X server to run and you should have that functioning before starting the installation.
The dependencies are just a few small packages (libSDL mainly and a few other libs) and on a straight Ubuntu install the base RetroPie install should be less then 800 Mb. However, since installation on a PC will compile everything, quite a few -dev packages will be installed (mostly X related) and take space during the installation itself - you can remove them after installation is finished.
If you add more emulators, the RetroPie installation might take more space, but not considerably. I expect the ROM folder and the scraped artwork (video/images) to take (a lot) more than a RetroPie install. If you managed to get your base installation under 5 GB, I think the installation to be fine and still have space to spare. Make sure you have a supported distro - Debian/Ubuntu.
For anyone who might want to give this a try: I just picked up a used Dell Chromebook 13 and with GalliumOS on it, RetroPie installed using the Ubuntu instructions very easily.
Things run very well, though GalliumOS seems to want to turn on the screen saver even if I'm using my USB controller. The workaround was to go into the power manager and enable "Presentation Mode" so the screen doesn't dim.