Create a request to get your total amount of ETH and use that value. Either that or track all your orders and add them up.
To answer your question a bit more specifically, I don’t think there is an sell all value that you can pass to the create order endpoint. If there was I think it would say here. So you have to list accounts to get your total amount and use that.
I might be wrong about this. If there is a sell all parameter please tell me because that would be useful.