I’m not actually sure what the limit is. I think cards up to 1Tb should work as long as they don’t change their protocol in some weird way. There may be limits within fat32 that prevents larger file systems, but I don’t actually think so. I have personally tried 128Mb cards, which work just fine.
Larger cards tends to use smaller flash cells and more bits per cell, which can lead to slower read/write speeds, but as technology improves, this issue starts to go away. It used to be that cards that were 16Gb or smaller were more reliable, but I’m not sure that’s the case anymore. I suspect that the new sweet-spot is somewhere around 64Gb.
My recommendation is to look for “high endurance” or “industrial” SD cards. They may not be the fastest cards on the market, but they seem to have less problems in the long run.
Ideally, SD card manufacturers would publish TBW values in the same way that SSD manufacturers do, as that would give an indication of how “durable” a particular card is, but unfortunately that is not common practice yet.
There is nothing in ProffieOS that makes large SD cards slower. (that I’m aware of) So if you find a large, fast, cheap and reliable sd card, there is no reason to go look for a smaller one.