According to Marty Stratton, id Software studio director and executive producer, Doom Eternal is "about double the size of Doom 2016".
“It’s about double the size of Doom 2016. It’s epic; it’s the best thing we’ve ever created at the studio. Our levels are bigger than they’ve ever been, we’ve done double the amount of demons, so from beginning to end you’re gonna see and experience things that are always new. I refer to this as the game version of a page-turner book.”
https://www.pcgamesn.com/doom-eternal/how-long-is-story-length