Figure ratio is indeed a good place to start as 79thPA states. I use 20:1 for my War of 1812 units. Also, You have to decide what type of militia that you are representing.
Arguably there are three types of Canadian militia that operated during the War of 1812; Sedentary, embodied (flank companies) and fencibles. I am guessing that you are likely representing the sedentary, which had basic training and tended to stay in the county that were raised.
If there was a major threat to the county or region (for example the American invasion of Queenston Heights and the Lincoln Militia Regiment) various companies representing the various districts of Lincoln county would gather together by their representative companies to form a militia battalion. As their regular counterparts the militia battalion would have its own command group sometimes with their own colours.
Generally, the sedentary militia fought as close order line companies but the well trained 'select embodied' flank companies fought in close order and skirmish as dictated by the tactical situation.
As to presentation on the table top battlefield at the 20:1 ratio I make most of militia and regular companies four figure companies, which reflects the average size company size of 80 men. Of course there were regular and militia companies that were as large as a 100 men and as small as 40 or less.
So that is the briefest answer to your question that I can give. However, please do not hesitate to ask further clarifying questions.
looks at a multi-period building.