https://wccftech.com/fallout-76-beta-download-size-preload/

Bethesda has revealed the Fallout 76 beta download size and has confirmed that it won’t be available for pre-load.

We already covered that Fallout 76’s BETA will include the entire game, and that progress made in the B.E.T.A. will carry over to the full game. Through the game’s updated FAQ page, publisher Bethesda has now also revealed the BETA’s download size – approximately 45GB.