Hi vschitturi,
I know of no command, once a filesystem is created, that will increase the size of a directory.
Files (contents) are identified not by their name which is recorded in the directory file, but by what is called an inode number also recorded in the directory. It is possible to "run out" of inodes. When this happens, you cannot add data to the device, even though there may be free space available. Read about inodes
here. There is a link at the end of that inode webpage to an article on the topic: The Linux Virtual File-system Layer: Inodes and Operations which you should also read for familiarity.
When this occurs, you may have to backup the contents of a partition to another disk, resize the partition given there exists more space on the original disk, and then restore the backup into the newly resized partition.
-- Tom