That’s an interesting question! I’m currently using a SanDisk Extreme 256GB card with my CM4 uConsole. I installed Raspberry Pi Diagnostics using sudo apt install agnostics and received the following results.
Sequential write speed: 36,551 KB/sec
Random write speed: 1049 IOPS
Random read speed: 2700 IOPS
It’s well below the 130 MB write speed that the card is rated for, but seems to be in line with other benchmarks made using Raspberry Pi 4B’s internal SD card slot. My guess is that it’s a limitation of the CM4 SD card interface. If you’re using the uConsole’s SD card slot I wouldn’t overspend for the fastest card on the market.
As for capacity, ClockworkPi lists a tested maximum capacity of 256GB on their website but other users have reported larger cards working. I think you’re probably safe so long as you don’t go over 2TB.