Coinbase is not a true exchange. It's more of a broker. Their exchange offering is called GDAX, and it supports daily withdrawal limits in the tens of millions of dollars for verified users (possibly higher, I don't know an upper limit). The initial limits are lower, but if you have the money and aren't doing anything illegal you can simply request higher limits. Also, deposits and trades are completely unlimited, so you can sell a billion now and withdraw later over time.

Other exchanges with high USD volume include Bitstamp and Gemini. Bitfinex is also an option if you trust them and live outside the US (I don't).