Is it safe to put a 256 in a mavic pro?

According to the specs Mavic Pro only supports up to 64 GB SD cards.

Thank you I thought I read that but was sure.

Wasn't sure

Yup. I have a 64 gig in mine and always have plenty of room.  I think that 64 gigs will hold far more than your batteries and flying time will contribute.  One thing I found is that, if you delete videos off your card while on your computer, it puts those videos into the trash of the SD card.  So, I always make sure to empty the trash while my card is still "live" in the computer.  

Not sure if this will be the correct answer you are looking for. I had it in my notes that I keep on the Mavic.

In summary, there are three classes of SD cards:
SD, up to 2GB in size, formatted in FAT12 or FAT16
SDHC, from 2GB - 32GB in size, formatted in FAT32
SDXC, from 32GB - 2TB in size, formatted in exFAT

Regular SD cards, and micro SD cards are capable of any of these classes. SD card class support is backwards compatible (e.g. SDHC cards are supported in a device that supports SDXC cards), but not the other way round (e.g. SDXC cards are not supported in a device that only supports SDHC).

Because a device that supports a 64GB card must already support SDXC cards formatted in exFAT, I don't see why a larger card wouldn't be supported either. But, a card that supports up to 32GB of size only supports SDHC cards, and therefore not capable of using the larger SDXC cards.

Please remember though that there may still be devices that will not support over 64GB SD cards (even though they theoretically should), possibly due to hardware or software limitations.

10 mins of 4K video is around 8Gb...  I've found 32 or 64 plenty...  It will almost certainly handle a higher capacity card but I'm not sure why you'd need one... cards corrupt and you don't want too much on there without being transferred to your laptop or PC

Ian