I have a R5 which uses a CF Express B card in 1 slot. What’s the maximum size catd you can theoretically use as I see on Amazon that there are cards which are 1 TB or 2TB in size?

Angelbird has a 4 TB card so that’s the max right now.

I don't know about theoretical limits.I see 4TB cards:cf express type b | B&H Photo Video (bhphotovideo.com)Merely $1.8k US each.As CF Express Type B cards are PCI-E/NVME, the addressable size limit may be much larger than any plausible card size.