Sizes 8" x 40" and 14" x 47" are worn on the neck and hanging down in front, whereas Sizes 30" x 51" and 43" x 51" are worn according to the traditional tallit style, draping down the back, or wrapped around the sides and held in front, according to the Yemenite and German customs.
The Size 30 x 51" generally fits boys 4-7, while the 43 x 51" size typically fits boys age 7 and up.