How to add Huge Database option in FM19
Extract the contents of the downloaded .zip file.

Move the folder "panels" to:
\Documents\Sports Interactive\Football Manager 2019\ (or corresponding user data folder if you changed it)

Open Football Manager 2019 and go to Preferences > Interface.
Click the "Clear Cache" button. Return to this screen.
Enable "Reload skin when confirming changes in Preferences".
Click the "Reload Skin" button.

Launch a new game and the huge database option will be available in the size database menu. It also works in the advanced database options.

How to use use Huge Database in FM19
• From FM19 start screen, select new career game and choose the type of save.
• Choose "Advanced Setup" to arrive to the Active Leagues screen.
• In the top right area, open the drop-down menu below approximate player count.
• The choice "Huge" should appear after Small, Medium and Large.
• You can also use the "Huge" option in the Advanced... database settings menu.