Your brochure's main job is to give you enough space to go into more detail about your services and offerings. To answer the questions that you most commonly get asked about your services.
It should also be a portable size, something that you won't mind carrying around. And, a size that your customers will take home, or back to their offices, and that will give them something tangible to review and remember you by.
And, the design and print quality of the brochure should show that your company is established, and that your services are set up in such a way that you'll be offering them for some time. The very fact that you have committed them to paper, and invested in getting the details printed up, means to your customers that you are established.