Hey there!
I'd reccomend leaving the difficulty/aggressiveness at normal.
The galaxy size will dictate the length of the game - if you want a quick one, go no larger than small. If you do this, i'd avoid advanced AI starts incase you start next to one. Same thing with fallen empires - on a small galaxy, i'd disable them entirely.
If you want a longer game, crank the galaxy size to medium or higher, but just be aware the game is gonna be long! Feel free to throw in advanced AI and even fallen empires at this point. If you're playing co-op, you probably want to set the starting positions to "Player cluster" so you start near eachother. Also end game crises are fun, but if you're both new to the game, they might be a bit much... depends how brave you're feeling
Galaxy shape is just up to your tastes. There's arguments to be made about the strategy of certain FTL types and how they interact with the different galaxy types, but I wouldn't worry about any of that for now.
Good luck!