Architects of large enterprise networks, how do you handle the sizing/categorization of sites?

In any large environment, I'm sure that there is quite a bit of standardization that needs to occur in order to keep things clean and scalable. When it comes to standardizing the design of new sites, how would you typically determine which design the site will be built using?

I imagine that it is some combination of user count, number of IDFs, and how critical it is to the business. But I'm curious to get a bit more detail as to how that process works. I'm in vendor-land today, and when I last worked for an enterprise I wasn't involved with their design process; so I'm not super familiar with how this would typically be approached.