According to the FAQ there are no limit within the specs. I haven’t tested it yet.
SD CARDS AND STORAGE
What minimal size of SD card do I need?
No size limitation on our end, we will support any SDHC card that meets the specifications.
What max size of SD card can it support?
No size limitation on our end, we will support any SDHC card that meets the specifications.
Having said it, in other SoC forums, like https://forum.pine64.org, there are a lot of people running into SD card quality issues. Not every USH-1 / class 10 cards are the same and worked well with that platform.
http://elinux.org/RPi_SD_cards#Performance gives some information how to test read/write speeds with sdcards.
