The Final Fantasy XV PC demo is now available to play and is 21GB in size. So, if you would like to get it, you can grab it on Steam, Origin, or the Microsoft Store.
Furthermore, Square Enix say the demo consists of the tutorial and the "opening main quests," This means it is likely give players a sense of the game's combat and, and see how well it runs on your computer. If you're interested in benchmarking, Square Enix have already released a tool for Final Fantasy XV to let you do just that. Final Fantasy XV: Windows Edition releases on March 6th 2018.