What size is the space heater? This will tell you how many BTU/hr you need to deliver to that area.
HVAC design is more difficult than electronic circuit design, but if you're up to it have your local library borrow Manual D and Manual J through interlibrary loan. The lookup tables solve the equations for you.
How many therms/month do you use [it's on your gas bill]?
What are your HDDs?
Heating & Cooling Degree Days - Free Worldwide Data Calculation
Looks like 1300 for last January.
You also need
The Stack Effect is in your favor for heating, on this one. Cooling is another story.