When you send an Ethereum transaction, it costs a small transaction fee, paid in units called "gas". For this reason, it's impossible to send a transaction for an account's full balance, you have to leave an amount for the transaction fee.
Unfortunately, MetaMask has not yet implemented a feature to automatically send your max balance, minus the gas fee, so sending an account's full balance would take some manual calculation.
An easy way to send your full balance in the meanwhile is to export your MetaMask account to MyEtherWallet, which has a "send max" feature, and send to the exchange from there.
To export an account, click the key icon on your account detail screen. This will allow you to copy your private key, which you can use to import that full account into any other ethereum client.
Sorry we don't have that feature ready yet. You can track our progress on it here: