A 10-person meeting room feels bigger on paper than it does at the whiteboard. Once everyone is seated and the agenda gets visual, a standard board fills up within minutes, and someone starts erasing ideas just to keep the session moving. Sizing the board to the room, not to whatever wall space happens to be free, is what actually prevents that. The table length is a reasonable starting point, but it rarely tells the whole story on its own. This distinction becomes more important the larger and more expensive the intended board, since correcting a sizing mistake after installation is far more disruptive than getting it right during specification.
