Here is how you can import the NixOS module and Home Manager module respectively.
When imported, new options will be made available under services.flatpak. Please refer to the Module definition for configuration options.
Warning Your setup must be able to hold the size of your flatpak installation at least two times. This module will keep the flatpak installations from the previous successful boot in order to not make your flatpak apps crash. Each "generation" will be deduplicated.
If you have >8GB of free space in /var (or your home directory if you use the home-manager module) everything should work flawlessly (unless you use massive flatpaks, then account for these too).