Depends how much footage you want - the Mavic Air writes 4K/30P footage at 12.5MB/s.
1 Hour of footage = 45GB
30 Minutes of footage = 22.5GB
15 Minutes of footage (roughly one full flight) = 11.3GB
In my opinion 64GB is the sweet spot because you can run through all 3 batteries of the Fly More combo before you need to offload or replace the card, but you could do 32GB or 128GB if you choose. Personally I don't like to put too many eggs into one basket in case something happens and I lose irreplaceable footage.
More importantly you need to make sure you get a card with a minimum sustained write speed that can handle 100Mbps / 12.5MB/s. You cannot simply look at the card's transfer rate, which is often presented in a highly misleading way while shopping. What you need to look for is a "U3" or "V30" designation, guaranteeing a minimum sustained write speed of at least 30MB/s. Class 10 / U1 / V10 cards are not fast enough (10MB/s write).
I use these cards, but anything with a similar write speed rating from a major brand (Lexar, Samsung, Sandisk, etc.) will be fine:
change card when changing battery or download it quickly to a laptop with a cable or download to skypixel. I haven't lost footage due to a lost drone or a crash but I have hit the capacity during a shoot and that is frustrating.