Anything smaller than that, or theoretically larger, will not give you the stability that a solid confession of faith will do.
For example, you might say, “The Bible is our confession.” But that wouldn’t help us. All that tells us is that the contents of this book constitute our confession. The real question is, “Tell me what are the contents of this book are.”
A really good confession of faith is a summary of the theology of the Bible, written, of course, in a particular historical context but with a view to the people of God in every place and in every generation.
For example, in Intervarsity in the United Kingdom, their ten-point confession of faith was for me a terrific nurturing organization. But the value of that ten-point confession of faith pales into insignificance with something like the Westminster Confession of Faith.
The Westminster Confession of Faith will do you as the one book you have on a deserted island. It will do you if you’re an elder or pastor as a foundation for virtually everything you encounter in the life of the Christian church, including principles for applying the gospel to situations that the Westminster Divines never contemplated would exist.