Basically, the allocation unit size is the block size on your hard drive when it formats NTFS. If you have lots of small files, then it's a good idea to keep the allocation size small so your harddrive space won't be wasted. ... In terms of space efficiency, smaller allocation unit sizes perform better.

Does allocation unit size affect speed?

Would this actually speed things up? The answer is likely no. So far there haven't been any real-world examples of larger allocation unit sizes leading to any performance changes on SSDs, even though theoretically this could be the case due to the lower complexity of your file system and faster read-write speeds.

What will happen if the allocation unit size is large?

If you make the allocation size too big, you'll waste space as many units end up partially full. Make them too small and you end up with files split across many different allocation units. Although modern hard drives are so large that concern about wasted file allocation unit space doesn't really matter anymore.