When putting a shower niche in an existing wall, 12”w x 24”h is a good size. It allows the niche to fit between studs which are often 16 inches apart. With a glass shelf at 12 inches this creates 2 feet of shelf space. The typical depth of a shower niche is 3.5 inches, the width of a wood stud. In a high rise building, the steel studs may be 2.5 inches deep and this should be considered. This is still wide enough for most bottles.