It's best practice to download anything you want more or less after each use, then format the card in camera from the app.
There's no real need for more than 32GB, that would be enough for three batteries worth of flight on highest setting.
They are relatively inexpensive so easy enough to have a second one in the case, this is a good plan anyway if (like me) the usual plan is to take out the card and try to remember not to leave it in the reader.
Note you need more or less the fastest card out there for the 100bitrate recordings.
I quite like Toshiba but Sandisk is arguably the market leader.

The Mavic 2 has 8GB of storage onboard but that often won't be enough unless you need just one more flight (or forgot the card...)