How large should the work be?

The rules behind the planner

Above furniture, a work looks settled when it is about two thirds as wide as the piece below it, and never wider. Its lower edge sits 20 to 30 cm above the furniture. On an open wall, the centre of the work is placed at about 150 cm, the eye height of a standing adult, and the work takes up about half to two thirds of the wall width.

These are starting points, not laws. High ceilings, long viewing distances in lobbies and restaurants, and seated viewers change the ideal size; the viewer on every work lets you check the result in a real room.