Regarding size, I did a regular debian 10 netinstall. And when running the xoce installation script after the installation, there was an additional 230 MB of packages and data downloaded.
That means a total of 350+230 = 580 MB have to be downloaded to install the OS and build XO from scratch.
Debian iso and packages downloads very quickly as they are sponsored by a CDN (Fastly) that delivers their files.
Anyway starting from nothing, it took about 7 minutes to download everything needed, install debian unattended and build xo with the script and have everything up and running.
Some of the steps I did manually like create the VM because we haven't fully automated everything yet. I only took actual running time for the server in account though.