You need to do some simple maths.
Rules to follow:
1. Try not to have your seating distance nearer than 1.5 * screen width. My personal setup is 180cm wide screen viewed from 280cm.
2. From your seating position, the angle to the edge of the screen should not be more than 30 degrees. Simple trigonometry can calculate this EASILY eg:
Angle = tan-1 (w / 2d) where w = screenwidth in cm and d=distance to screen in cm.
This angle should not be more than 30 degrees otherwise you will be moving your head around to see the picture.
3. Find out the throw ratio of the projector. This will generally allow you to calculate the image width for any given distance from the projector to the screen. Some throw ratios refer to image diagonal. If this is the case you will obviously need to calculate the diagonal of your screen.
This is very simple. Use Pythagoras ie:
Diagonal = SQRT ( (width * width) + (height * height) )
Apologies to mathematicians out there but this was the simplest way I could think of of expressing it....
If you know the width of the screen and the aspect ratio you can work out the height eg:
Screen width = 180 cm
aspect = 16:9
height = 180 * 9/16 = 102cm
The idea of the throw ratio is so you can draw a diagram, having your proposed screen on it and proposed projector siting. Then make sure the projector is actually capable of projecting the image that you plan onto the screen at the correct size.
In addition, if you want to get more technical, you can then work out the illumination level of the screen if you work out it's surface area and then you can tell how "bright" the image is going to be. This is handy in choosing a projector - you don't need a 2000 lumen light cannon to satisfactorily illuminate a 80" picture. However if you plan a 120" picture than you will do.
At the end of the day, the whole issue is a bit more complex than just finger-in-the air guesses. Most people just guess and trust and hope to luck and a bit of experimentation. If you don't want to do that, then you're going to need to do some maths otherwise you might find out that your proposed projector siting, image size or viewing angle is not what you thought it might be.
Hope this helps
J