C19 was the smallest size that was supported by the CUDA miner;
it was never used in Grin testnet, just to help diagnose solver issues.
C30 was used as part of the primary Cuckatoo30+ PoW of testnet4,
and was retired when testnet4 was replaced by floonet in Dec 2018.
C32 and C33 are both part of the mainnet primary PoW Cuckatoo31+, as are C34 through C64, all of which could be used right now, but anything beyond C32 is too hard to solve efficiently with current day hardware.
C32 will become more interesting next year if ASICs have not arrived yet and C31 starts getting phased out (which will take around 7 months altogether).