You need to be more specific about a number of things. What is your desired temperature? How large is the space? What are the walls and roof made of? What type of lights are you using? A decently insulated house needs approx 12000btu per 1000 sq ft just to keep it comfortable during the heat...
I know I'm a bit late to the party but thought I'd add some info for future searches. Some considerations for installing a ductless split AC system that will be running in during cold seasons:
Most ductless split systems are designed with some ability to operate at low outdoor temperatures but...