There aren't really any specifications I've ever seen, but in general the oldest, slowest, smallest microSD card you can find will be the one most likely to work. Anything over 2GB is wasteful. If you try a new class 10 256GB card it's almost guaranteed not to work. Roku sells a 2GB card on their site, but it's currently out of stock: https://www.roku.com/products/accessories/player
Keep in mind that it only increases channel storage and won't have any effect on streaming whatsoever.
Roku Community Streaming Expert
Help others find this answer and click "Accept as Solution."
If you appreciate my answer, maybe give me a Kudo.
I am not a Roku employee.
