One of the most important tasks for a web developer before any work is begun on the page is the conception stage. After a concept has been developed, it is important that the developer thinks of his or her website in realistic terms.
For example, if a database will be necessary, consider how much space an entry will take up, and how many entries there will be total. Think of the hardware requirements of your site. Think, using what knowledge of programming and backend you may already have (or consulting a professional), if what you are trying to do will even be possible.
Most importantly, keep in mind the limitations of your human resources. You should have an idea of what size operation you're going to be running: money, men, etc. You don't want to stretch your people too thin. If you think it may be necessary to scale down your project to make it more realistic from a human resources point, then do it. It's better to be overcautious than to work your men and yourself into the ground.
Also, mind your capital and other resources. In Section 3.1, What's needed to develop a website?, we'll be covering this more in-depth. For now, just use common sense. One person didn't make Amazon.com on one dime in one day.