~1.4GB for GNOME, ~1.9GB for KDE Plasma. Once the download is completed it usually takes less than 20 seconds to deploy on reasonable specced hardware.
If you can maintain a reasonable download speed such as 15MB/s, it is much faster than rpm-ostree, even though it is much heavier on bandwidth. rpm-ostree typically taking 5+ minutes to do a deployment, Arkdep does it in 2.
For apps you will rely on Flatpaks and Podman/Distrobox. Unless you want to build your own images, in which case you will have full flexibility on what you ship with your image.
You can at any time invoke pacman which will prompt you to unlock root and install stuff if you must. But these changes are lost when updating.