By default 8Flight attempts to download everything for your available regions (currently the U.S.). In Profile->Downloads/Manage Data you can select which enroute charts (VFR, IFR Low, IFR High, and/or Helicopter VFR) to download and for which region (currently the U.S.), as well as select specific states to download instrument procedures charts for (SIDs/STARs/Approaches).
Downloading all available data currently requires approximately 12 GB of space on your device. Downloading the minimum data set currently requires approximately 3-5 GB of space on your device.