What is an A-19 light bulb or lamp?
It defines the physical size of bulb that can be accomodated.


  The most commonly used A-series light bulb type is the A19 bulb, which is 2 3⁄8 inches (60 mm) wide at its widest point, approximately 4 3⁄8 inches (110 mm) in length, and has a one-inch wide (type E26, i.e. approximately 26 millimetres wide) Edison screw base.


From Wikipedia

Any CFL that fits in the space provided should work

Some LED lamps sometimes need airflow to keep their temperature in operating ranges. So might have issues in a fully enclosed fixture