diskutil info /dev/disk4s1 | grep Free
   Volume Free Space:        58.0 GB (58048249856 Bytes) (exactly 113375488 512-Byte-Units) (90.9%)


58048643072 (58,048,643,072) bytes free

DJI says the Spark's max bitrate is 24Mbps
that's 24/8 = 3MBps

echo $((58048643072/3/1024/1024/60))
307

that's 307 minutes (5 hours) of video for my 64GB card, ignoring file overhead


the above are for after HCC Office was copied on.  prior to that, the free space was 63820267520 bytes

echo $((63820267520/3/1024/1024/60))
338

so actual full time for a 64GB microSD card is 5 hours 30 minutes

expected time for 32GB is 2:45 (again, ignoring filesystem overhead)

these estimates are based on MAXIMUM bitrate, which may be significantly more than required for
typical videos where significant portions of the frame aren't changing




Originally, DJI stated the card size limit was 32gb
One source I found recently (https://unlimitedrone.com/2018/03/29/best-micro-sd-cards-guide/) claims 64GB
DJI's current specs page (https://www.dji.com/spark/info) also states 64GB
I have seen 128GB stated at least once (unconfirmed)

Spark's video is rated 1920×1080 @ 30p  ("1080p") and DJI recommends a class 10 or U1 card for clean streaming
The Mavic's "4k" video recommends a class U3 (U2 is specified as barely adequate)

DJI's stated supported filesystem is only FAT32, but other sources also say exFAT is supported as well (VERIFY THIS)
Most DS cards come pre-formatted Fdisk/FAT32 (<=32GB) or Fdisk/exFAT (>32GB)
FAT32 cannot handle individual FILES greater than 32GB, but max disk space is 2TB, so FAT32 does suffice in most cases


I am assuming Spark does not support HFS or NTFS
I am also assuming Spark only supports partition schemes Fdisk and possibl MBR, but not GPT

Several sources recommended reformatting a new card using the Spark itself to insure maximum compatibility.
Spark will format using Fdisk/FAT32 regardless of card capacity, unlike Mavic that will format exFAT for >32GB cards


Mavic Air states a max bitrate of 100Mbps (the camera's raw bitrare, assuming 24 bit color / bits per pixel, is about 6000Mbps)
Mavic Air video resolutions:
4K Ultra HD: 3840×2160 24/25/30p
2.7K: 2720×1530 24/25/30/48/50/60p
FHD: 1920×1080 24/25/30/48/50/60/120p
HD: 1280×720 24/25/30/48/50/60/120p
echo $((3840*2160*30*24/1000/1000))
echo $((2720*1530*60*24/1000/1000))
echo $((1920*1080*120*24/1000/1000))
echo $((1280*720*120*24/1000/1000))

Mavic Pro max bitrate is 60Mbps  (https://www.dji.com/mavic/info?pbc=mF6h4ZTt)