Oldcastle retaining wall blocks
Oldcastle makes them under a license from one of the several different licensors.
The license may have requirements on what sizes are sold, because often the larger sizes are frequently mis-used in higher walls by DIYers.
I assume you are referring to the length of the units or is it the thickness of the walls? The height would be somewhat less than 8". there are many different sizes made in the world, but for walls higher the 4' or 5' (up to 40'high), they must be engineered. Larger size units may be available elsewhere, but there usually is no technical support on the use.