2x2s or 4s is same thing. I've heard that 2x2s can be more convenient. Like if one cell goes bad you only have to replace a 2s and not the whole 4s.

For size not sure but in the picture they show it using 2 hardpack. Most people use hardpacks in cars/trucks because they are more protected in a crash.

The hardpacks are usually around 130-140mm long.. Thickness depends on the capacity though.

Here are the cheap hardpacks