Howdy....

Mine’s 12’ wide by 16’ deep and it’s too damn small. My next HT will be 16’ wide and 24’ deep.

You will have to decide if you want one or two rows of seats first. But if you go with a single row, I’d still make it a minimum of 20’ in length just so you have room for proper 7.1 speaker placement, furniture placement, your equipment and storage.