Well, the best advice when building for your own class is to set up obstacles that make it a challenge and limit to how far. If your wondering about sizes, the sentry gun's limitation of view is 1024. If the map is below that, a well placed sentry could cover the entire courtyard and possible only passages into the bases. 2fort, as said, for example has the sewer areas which allow opposing teams to enter the base an alternative way.
Given battle areas of the map, for battle are around 2000-3500 on some maps like Gravelpit, Dustbowl. Usually the area is broken up with buildings in the way. In Gravelpit for example, the blu sniper's cover is limited from the concrete tunnels from their base. They'd have to leave the 'safety' of tunnel have more range across the map. In Dustbowl, the beginning area may seem like a sniper haven for the first checkpoint of part A, but the attacking team has a clear shot to the checkpoint that the sniper can't cover from the save haven of the tunnels.