1. Do a stereo musical sound check. Yes, even if this is a cinema theater. Do not use a subwoofer yet.
Put speakers 1/3 the length of the room along the short wall. Put a chair 2/3 of the length the room. Spread speakers apart so that you get an equilateral triangle with the listener. Best effort.
Fine tune speaker placement for focus, good tonal balance, good bass. This setup will be the foundation of your HT speaker setup.
2. Having determined the position of your chair and its distance from the front wall (short wall where you will mount the screen), work out the size of screen you need using web calculators. You do not want to see pixels. They should merge into a clear video.
What is your room dimension? (length, width, height in feet)