If security is not an issue, you might consider using exFAT instead of NTFS.
It was designed for thumb drives and doesn't carry the overhead that NTFS does.
I know this doesn't answer your Cluster size question, but it's something to think about.
Speaking of cluster size-
In the old days, Norton Utilities had a slack space analyzer that would show you what the optimal cluster size to slack space ratio should be based on your current disk usage.
Does anyone know of a current utility that will perform that same function?
(I don't mean to bump here and if this should be in a new thread, then somebody holler at me please.)