Screen size should begin with seating distance, room width, aspect-ratio strategy and the type of content the room is being designed around.
Start with the seats
A screen is not large or small in isolation. The image is experienced from a particular seat. We therefore prefer to establish the main seating position and likely rows before fixing image width.
Width matters more than diagonal
Dedicated cinemas are often easier to discuss in image width because projection throw, screen-wall construction, loudspeaker placement and masking all relate directly to width.
The projector and screen are one decision
Image width affects required light output. Screen material affects both light and sound. An acoustically transparent screen changes the loudspeaker and screen-wall design. Masking changes how different aspect ratios are experienced.
The useful question is not ‘what screen size should I buy?’ but ‘what image geometry belongs in this room?’