Based on the current order book at Kraken you could probably sell off about 1000 BTC and it would drop the price about 5%. Poloniex looks to be about the same.

Based on the current order book at Kraken you could probably sell off about 1000 BTC and it would drop the price about 5%. Poloniex looks to be about the same. So across all the major exchanges maybe you could sell of 5-10k BTC at once, without dropping the price too much.