I can confirm 256 Gig cards work but they either have a Problem formatting as Internal storage, or just take a REALLY long time to go from 20% to 100% as it took well over 30 min then ended with a some kind of JAVA error displayed.
As Portable storage, no problem. So use that option.
You will have to use Windows or Linux to reformat as ChromeOS needs to be in developer mode to have that option to do so if it goofs up. Its also best to use the NON smart OS cards as they work better with ones unlike some of the Picky Sandisk ones that have their own OS on board the card that can play havoc on Android (but not always)
Phone works great with what I think was a Generic 256 card from aliExpress.
Weird that you can get a 1TB SD Card for about 30 bucks today on Amazon. (Its slower but still cheaper then a HDD stroage!)