Out of curiosity I built a distribution of file sizes in my blobs/ directory:
1k: 190668
2k: 101621
4k: 88516
8k: 66608
16k: 61292
32k: 60998
64k: 62332
128k: 77795
256k: 123921
512k: 53416
1M: 219292
2M: 1293863
4M: 3335
8M: 16809
16M: 811
Which suggests that pieces can be as small as 1-2 kb and max size for my node is so far 20995328 bytes.