Once you have an idea of how you'll be using your heater, the next thing to consider is exactly how much power you'll need to expand in order to meet your heating needs. It's better to have a larger heater and not use it, but there's a pretty basic ratio that you can use in order to quickly judge whether a heater is properly suited for your space or not.
The Heating Ratio
Total area (sq. ft.) x 10 = Required Wattage
The ratio above, in layman's terms, means that for every 10 feet of square space in your room, your heater requires 10 watts of power, especially if that heater is the primary heat source in your room. If you have a solid central heating system, then the ratio becomes more forgiving.
The opposite is true if you have a high ceiling, like a theater or a large dining hall. Higher ceilings mean significantly more air to heat, so multiply your normal ratio by 1.25-1.5 in order to find a better estimate.