Re: Producer liked my script, need advice.
Yes, you need a bible. Unless your plan is to never sell your show.
Also, I don't know where people are getting these page counts from, but just as an example, the bible for THE WIRE was 79 pages. FREAKS AND GEEKS was 55. So 20 being huge isn't accurate. Although with that being said, they vary wildly. TRUE DETECTIVE was only 10. LOST was 27, I think. As far as rules go, series bibles probably have the fewest adhered to of almost anything in the industry.
Basically, you want to introduce your series. The series, not just the pilot. The idea of the bible is to convince people that there are multiple seasons worth of episodes for your idea. Every single bible I've ever seen includes some sort of introduction, character descriptions and specific episode ideas. Many also include set details, because every TV show has sets which they reuse to keep costs down. So maybe a primary set and secondary set section, if applicable.
The more concrete you can make things, the better. So if you can say, "this is episode 2, this is episode 3, etc." all the better than just randomly throwing in some ideas for an episode here and there. Like in THE WIRE bible, they've got 3-4 pages for each episode, essentially entirely outlines for every one, even including specific dialogue that would be used. Same goes for including details about a character's specific arc throughout one or multiple seasons. If you can, then great.
Details, details, details.