The golden ratio for home theater design is widely considered to be H (Height) x 1.28H (Width) x 1.54H (Length/Depth). For example, if you have a room that has a 10ft ceiling height, the ideal width and height would be 12.8ft and 15.4ft respectively.

While there is no single perfect size for a home theatre room, the rule of thumb is the larger the space you can allocate, the better. For the best viewing experience, a room should ideally be rectangle, approximately 6m long by 4.5m wide and with high ceilings.

All things included, the average cost of a home theater is between $10,000 and $50,000 with the mode being around $17,000. It's important to keep in mind that this is just an average and the cost could be higher or lower depending on your specific needs and preferences.