The size of a file system, as used for
mke2fs and other similar tools) is the external size of the file system, i.e. the amount of space it occupies as viewed from the outside. The internal data structures of the file system don’t matter here. Everything in the file system — metadata, data, and unused space within the file system — fits within the file system’s size. This also includes any boot loader hosted in the same partition as the file system, where such a configuration is possible (it is with Ext2/3/4, not with XFS, for example).
The best way to determine the current size of a file system is to read it from the file system; for example, for ext2/3/4 file systems, you’d multiply the block count by the block size:
dumpe2fs -h /path/to/file/system | grep -E 'Block (count|size):'
By default, the size of a file system is usually the size of its container, e.g. its containing partition or logical volume. This is the value that
resize2fs will use if you don’t specify a size explicitly. Specifying
size is useful when you want to shrink a file system and its container: you’d start by running
resize2fs with the target size, and then shrink the container (partition, logical volume, etc.) to match.
To determine the minimum size to which you can shrink a file system, the best approach is to use
resize2fs -M -P /path/to/file/system
will tell you how many blocks a minimal file system would occupy (preserving all the current contents); to determine the block size used, run
dumpe2fs -h /path/to/file/system | grep Block\ size
You can get the details of the calculation by adding
resize2fs -d 32 -M -P /path/to/file/system
The maximum size of a file system is usually the size of its container (partition or logical volume). A file system which is in a partition or logical volume can never be bigger than the partition or logical volume; there can be additional constraints which limit the extent to which a file system can grow. (File systems stored in image files behave differently,
resize2fs changes the file size to match the file system size, whether growing or shrinking.)