I started with 6x 1K storage disks, you don't need to start with anything bigger. Those (plus a few DSUs for bulk) served me for an entire 64x64 quarry. I've since added a couple more 1K and 4 of the 4k storage disks.
FYI, if you want to store a small number of many different items, use smaller storage disks. Conversely, if you want to store a large number of just a few different items, use the larger storage disks. Someone else did the math, but that's the most efficient way to use the ME storage disks.