I don't trust my memory to remember what size I bought last time. I thought I would stop growing somewhere between the ages of 18 and 20. I stopped growing taller, but not in other ways. No living organism stays the same for very long. Their needs, goals, and desires change.
Class sizes change.
Mission goals change. In other words, if a congregation doesn't know what its mission statement is, it won't know how to pick the pastor that can best help it fulfill God's will for the life of the church at that time. The measuring process can be done in as little time as a week, or it may take several weeks. It can be done in many ways. Survey forms, retreat plans, and assessment guides abound if a church needs help.