I'm pretty sure that the 250KB limit has never been broken to date.
block 187901 499,245 bytes
block 191652 499,254 bytes
block 182862 499,261 bytes
block 192961 499,262 bytes
block 194270 499,273 bytes
These are the biggest 5 blocks up to the checkpoint at block 216,116. Interesting that they're all 499,2xx bytes long, as if whoever is mining them doesn't want to get too close to 500,000 bytes long.