I just finished a retaining wall very similar to your proposed one. I used EPHenry brand block, which I believe is similar to what you are looking at.
Basically, I did not go to a home center. I went to a place that specialized in products for masonry walls and such, and simply gave them my measurements. They figured out what I needed, and delivered it I may have ended up with a block or two extra if I wanted to be picky, but since my wall tapered from 36" to essentially nothing, where I stopped a particular course was up to me, so I was able to absorb all the blocks that were delivered.
As far as block size, it depends on your taste, I guess. We had a landscape guy come and give us an estimate, and he suggested using a smaller size, but I, instead, used a bigger block and did the work myself. I did not use the biggest size (85lb/block) but I used something bigger than what I found at the home stores (60lb/block). It was quite a workout.
My blocks are 6" high. A shorter height would mean you could be more gradual with your step-down, if your wall will be tapered like mine. However. the bigger blocks go faster, and for us, were scaled perfectly. The bigger blocks, although more expensive per block, probably were cheaper overall than the smaller ones. The other thing I liked about the bigger blocks was that you could order capping blocks to match, which really finishes off the top of the wall. EPHenry also has a web site.