Sorry, BLandry, but I have to respectfully disagree. I think you can do reasonable estimates on important systems as to how big it can get. Calculating table and index size is almost trivial, but everything depends. If the company is a brand-new startup, then doing accurate estimates will be difficult, especially if the company takes off and ramps up at an unanticipated and explosive rate. But that reinforces Phil's point: test under heavy (or monstrous) load; it will perform well with a light load and you have confidence of acceptable performance under heavy load.

It's definitely harder to accurately forecast size and load for new projects, much easier for redevelopments and conversions.