Most auto strains are short to medium in height. The strain info on many seed sites will give you insight into how tall you should expect them to grow. I have never had one reach the size they predict. Using smaller pots, not larger than three gallon and applying LST will guarantee a smaller plant. I have found that none of the auto strains I have grown exceeded 24 inches or came close to the expected height. Add 24" plant plus an11 inch pot and 18" for the light and you have 53" or 6" less than you need. So, if you purchase seeds for short plants, you will most likely be OK.
I currently have 1 Gorilla Glue and 1 Northern Lights autos which are 21" tall and almost ready for harvest. Depending on your light, the 18" elevation seems rather arbitrary. I currently have my 300w. light 9" above the plants at 90% power, Check the manufacturers specs for recommendations