We extended our 50s brick veneer no problems, sadly our builder has retired. 5x7 would be fine for me, but depends size of gathering and furniture. We now sit close to bigger TVs due to better graphics

Don't forget to put the maximum amount of insulation in the wall, as you can't do it later. The maximum amount of bulk insulation that will fit between a standard 90mm timber stud wall is R2.5 (possibly up to R2.7).