16-20 teams is kinda big. I'm in one 16 team league, but most of mine have 10 or 12 teams in them. It's all based on personal preference, but any bigger than 16 and you start running out of good players to start, so I'd cap it at 16 and probably try something smaller than that to start off.
Starting lineups are your preference. I've seen anywhere from 1-2 QBs, 1-2 RB, 2-4 WR, usually 1 TE and 1 K, and you can throw RB/WR and WR/TE flexes in there if you want. You can throw defensive players in if you want. I like to usually have only 1 QB, 2 RB, and either 3 WR or 2 WR and 1 RB/WR flex. Again, it varies from person to person on preferences.
For football dynasty leagues, usually leagues just add bench spots to account for all the young, developing players that are now worth roster spots. It's typical to take the number of starters and add a few spots to get the number of bench spots you want. I prefer to have more bench spots in dynasties, but it's all personal preference. Roster size depends on your starting lineup. I'm in a few dynasty leagues that start 9 players, and we have 22 players on our roster, just to give you an idea.
One thing about the size of the league: it's always a challenge to find active owners. I've had to replace owners in just about all of the leagues I've run, and none of them are above 12 people. I'd try to keep the number smaller just so you're not constantly replacing inactive owners.
Good luck with your league and if you have any more questions, either I or another person can answer it. Don't hesitate to send me a pm if you feel urged to.
BTW, you might wanna delete your other post; the mods get kinda annoyed at people that post the same thing more than once.