The field I've used in benchmarks is GF(0xFFF00001) which has root of unity of 0xFFF00000 = 0x100000 * 0xFFF order. Since my code implemented only FFT for 2^N sizes, it was limited to 2^20 order for this particular field and thus 2^20 blocks total.

It can process larger amount of blocks in other fields (f.e. GF(2^64-2^32+1)). Also, I have implemented other FFT algorithms, which will be published soon.

I ahve chosen this field for the speed (and million blocks is more than other RS implementations provide anyway), but RS in GF(2^64-2^32+1) will be even faster and allows 2^32 blocks even with the current limited FFT implementation.