The general rule to be applied is that the screen should be of a width that is 2/3 the distance between the screen itself and the front row of the audience. To put it in another way, people closest to the screen should be placed at a distance of what is 1.5 times the width of the screen. When in doubt, use the PISCR standard, which provides metrics for measurement and requirements for minimum contrast ratios for rear and front projection audiovisual projected image systems.
The bottom of the screen should be approximately 100-120 cm above the floor in a room with a level floor and several rows of seats. In rooms with theatre seating or only one or two rows, such as a home theatre, the bottom of the screen should usually be 60-90 cm above the floor. Try to make sure that the lower part of the screen will be visible from all seats. Extra drop may be required to position the screen at a comfortable viewing level in a room with a high ceiling.
Screen height is generally determined by the width of the screen and the projection formats to be used. The height should not be exaggerated, but correctly estimated. Extra unused screen surface can cause issues with the screens mechanism and surface flatness, which will not fall under any manufacture faults clause.
In determining the correct screen size in relation to the audience area, the goal is to make the screen large enough so those in the rear of the audience area can read the subject matter easily, but not so large that those in the front of the audience area have a problem seeing the full width of the projected image.