Coinbase only allows you to sell directly into your Coinbase fiat wallet. However, there is no limit on the amount you can sell to your wallet. After selling to your Coinbase fiat wallet, you can opt to either withdraw funds to your US bank account or repurchase digital assets on the platform.
When initiating a withdrawal of a sell from your fiat wallet to your bank account, a short holding period will be placed before you can withdraw the fiat from the sell. Despite the hold period, you are still able to sell an unlimited amount of your digital assets at the market price you desire.
For a step-by-step tutorial on withdrawing from your Coinbase fiat wallet to your personal bank account, please refer to this article.
For more information regarding available balance and your hold period, please refer to this FAQ.