A first chapter makes promises to the reader. It makes promises about who is telling the story. In its tone, there is a promise. In its suggested scope, there is a promise. There is promise in the language, in the premise set forth, in the central question. While your first chapter establishes point of view, presents the protagonist, suggests scope and style, and introduces conflict, it also sets forth the writer-reader contract.
When you break the contract, you break the fictional dream, dragging the reader out of the story and back to reality. Your job as a novelist is to keep the reader as deeply inside the world of your fictional story as possible, for every minute that she is reading the novel.
What promise do you want to make? What is your contract with the reader? Write as much as you can now. You can always come back and change this later. As you begin to understand more about the novel you are writing, you will get a clearer picture of the promises you are making to your reader.