Individual storage room in commercial cellar

Hello,
I can’t get a definite answer on having separate storage rooms for a commercial cellar. The cellar in already divided into four bigger spaces and the store above has access to one of these spaces. The store owner would like to make individual storage rooms in this bigger space. I’m getting different answers on how big these rooms can bet or whether or not it’s allowed. They would be built with a 2hr rated partitions and metal door. Thanks in advance!