In general it works like this: The officially supported maximum size is typically what was available when the specs for the device (the reader) were written. But all that means is that nobody tested a bigger one. If the protocol stays the same, a bigger one should still work.
For example, my Galxy S3 officially supports 64GB, but has no problem with my 128GB card.
So, no matter what the official spec will say, just try a bigger one 
(And of course, all standard compression stuff will work on the Librem 5. Just not sure if there will be good UI from the start. Also keep in mind that 7z uses a LOT of CPU to get the good compression rates it gets…)