In air-based systems, it's very important to plan for a collector array of the right size for your needs. It's a difficult calculation for inexperienced installers because you need to strike a balance between the collector output, the amount of thermal mass inside the building and the rate of heat loss from the structure. If you're unfamiliar with calculating thermal mass and heat loss, it's going to be even tougher.
As a rule of thumb, a good start is to compare the heat loss rate of the building to the listed output of the collector array, bearing in mind that the collector will run for about four hours a day in winter (on average), so it needs to work hard and fast! Adjust the rule of thumb for the amount of thermal mass in the building – if there's a huge amount, you may be able to allow output to exceed heat loss by as much as 100%.
An alternative rule of thumb is to use collector surface area and the building's footprint, assuming the building is average. In a cold climate, aim for an array that measures 10% of the building's footprint; in a hot climate or a building that performs well thermally, aim lower.
As always, the best way to work out your requirements is to talk to an experienced installer who is familiar with your region's climate and all the calculations.