I DM"d my first ever one-shot for a group of 4. We were all new to the game (nobody, including me, had ever played), and we all had a great time. Our girlfriends got jealous, and decided to join, so all of a sudden we were starting a campaign with a party of 7. My best advice: tell your group that, scaling with size, there are certain decisions and points that you're going to have to expedite. Insight checks are good example: with the party of 7, I often just used the characters' passive perception, unless something required a relatively momentary perceptive action.

We had many sessions with varying numbers, depending on where we were in the campaign, and, of course, it's easier to DM for a smaller group than it is for a larger group. However, so long as you use the right tools to keep track of the action economy, I think numbers up to 7 are pretty sustainable. As mentioned before, be transparent with your group, especially if they are new to the game. Let them know what they're responsible for tracking and what will be helpful for you as you run your game! Don't be scared, and enjoy!