From Madrid to Cordoba via the fast train I understand to store your luggage for a day tour in Cordoba is at the bus station across the street. How large are these lockers if I have a large suitcase?
The way to answer this question is to Google "left luggage Cordoba" and go from there.
Since it is quite possible not to get anything or anyone that can give you detailed
dimensions of a locker, you'll want to check on offices where you can store your luggage
securely behind the counter, as a backup option.