Cluster size is nothing but the allocation unit size used while formatting the hard disk. The default cluster size is usually 4096 bytes. You can find out the cluster size of a hard disk by doing the following:
Go to Start and in the search box type cmd and hit enter.
Type CHKDSK followed by the drive letter and it hit enter. For instance, if the drive letter is G, it should look like CHKDSK G:
At the end of the report, you’ll find the bytes in each allocation unit.