Depends on the lowest frequency that you were designing for.In reality a true FR horn would be front loaded with maybe a smaller chamber on the rear (to provide two sided loading). If it was floor/wall loaded ( cant tilt it back to a great degree)and you were aiming for a true spherical wave at 50 hz then the x and y mouth axis would be 21.49". If it was just wall loaded then the min dimension for the x/y axis is 43". Thats why its so much easier to have a good LF woofer and a front horn, saves on space. I find it much easier to just use a BLH up to and around 360-380 hz and let the FR driver do its direct radiator thing. ron