The size of a Redis list can be deterimed using the LLEN command. If I have a four element list stored at the key my_list, I can get the size using:
LLEN my_list
which will return 4.
If a user specifies a key that doesn't exist to LLEN, it will return a zero, but if a key is used that points to an item of a different datatype, an error will be returned.