The advantage of D&D (over online gaming) is that players have a much greater range of options for solving their problems. You can give them an open-ended problem and have them make up their own plan for solving it. (Or, if you give them a problem with an Intended Solution, be flexible if they come up with some other solution instead.)
"What's a good open-ended 5e scenario for first-time players?" might be good as a separate question -- but it might also be considered too broad.
One disadantage of D&D is that sometimes you get players who aren't interested in solving the problem at all. People will say: "I'm roleplaying a thief who steals from the rest of you! That's okay, because I'm roleplaying, and this is a roleplaying game!" People will say: "I'm a druid, and I don't believe in civilization, so I'm setting fire to the village!" Make sure to talk beforehand about what sort of game you want to run. Try reading the Same Page Tool or something similar for background.
I agree with you that the rules should be de-emphasized. Try not to say things like "you can't get there because it's 30 feet away and your move rate is only 25 feet".
You asked about scale, so I'll suggest that the game is more fun with four players (plus DM) than with six.
Don't forget to bring pregenerated characters.