For part a), if you want to figure out the number of combinations there are for the sizes, consider this:
You have four modules, and five sizes. Each size of module is like a box. You can throw all four into a single box, or spread them out. Let a module be an x and a box be the space between vertical lines ||. Then, the five "sizes" could be considered |||| and the four modules xxxx. Now, each possible order of x|xx||x| tells us a configuration for the modules of various sizes. This example would be one module of the first size, two of the second size, and one of the fourth. So, how many possible orders are there? Well, you have 8 slots where you are putting 4 x's and the rest of the slots get a |. So that is different choices. So, there are options for part a), just as you calculated.
Now, for part b), there are
different possible configurations.