Most people just build what they need. Cost and space is always an issue. There is a tendency for things to grow as the project goes along Mine is now 2 m x 1.5m. Started off as 1mx.5m! If you build your machine with tight enough tolerances, there is no reason that a large machine can't do fine detailed work. A 4th, 5th, 6th, axis is just additional ways to rotate your cutter or your work piece to allow for more complex cuts.