The size of Empty Folder is equal to Zero. If an empty Folder is created with no data then the folder doesn't contain any space.
In any file system there is a particular space reserved. For example, if you purchase memory card of 16 GB then you will get memory card of about 15 GB only. Some portion of memory will be reserved for important operation such as folder/directory.
Folder/directory are used to store files. Folders themselves doesn't contain any data so their size is equal to zero. If we see technically, as folder need to be created, it must contain some space. Yes, it contain very less or insignificant space which will be fulfilled by the reserved memory in any file system.So, we wil not see any space occupied by folder in any file system.