I think that more important than length is blade style, and what sort of materials you intend to be cutting. Blade length obviously does still play a role (with 14"-18" generally being the sweet-spot) but you'll find that as soon as you start exploring patterns other than the classical Latin machete that things begin getting a lot more interesting.
For harder woods where you need axe-like properties but still want to be able to take out light brush, go with a broad-bladed model with the mass concentrated towards the far end of the blade to generate a more powerful stroke. Bolos and kukris are two examples of this, though there are others out there that are even better at the task--mostly ones patterned after cane knives like the CS Heavy Machete, Condor Beaver Tail, or Tramontina Cane Machete. A piece for this type of work
can be made of thicker stock and shorter if that is your preference (though definitely not mine) and retain a fair amount of chopping power without taking up too much space.
For lighter brush-like materials go for a longer blade, or one that hooks forward (if you can find one). The most important thing here being that it be thin. A thick blade will most commonly cause more flexible and lightweight targets to move out of the way rather than be cut. For this I rather enjoy my Condor Viking or Bush Knife, using the forward hooked blade. Other examples would be the CS Kopis or Magnum Kukri machetes, the longer Latin pieces from Tramontina, or the Brush Thinner from Gerber/Fiskars.
For general work a medium sized classic Latin pattern tends to do a good job, and hence is on of the several reasons for being the most commonly produced of all machete patterns (among others but we won't get into that here). Goloks, barongs, and parangs all make good general-use pieces which can vary from medium-hard to medium-light depending on the exact pattern.
With all of the non-brand/model specific pieces mentioned there is a lot of variance, and so my generalizations will, of course, not stand up under scrutiny on a piece-by-piece basis. Just because a particular machete has a particular footprint does not mean that it is necessarily suited for one kind of work rather than another. The blade thickness, exact blade shape, edge angle, etc. etc. all play a very important role in deciding the appropriate use for a machete. I personally prefer thin blades on all of my machetes as it allows for deeper penetration with less effort during the cut. If additional mass is required I prefer it to be directly behind the cutting edge, rather than to the sides. It's not like you're going to pry with the thing!
Not to run on too long, but another key factor lies in proper technique with the machete in order to fully maximize its cutting potential. In general I find it to work best if I begin with the hand at about shoulder height, the machete gripped with the index and middle fingers--the ring and pinkie fingers relaxed. Proceed to rapidly extend the arm forward while "squeezing" the hand (utilizing pressure from the ring and pinkie) to "throw" the point forward in a controlled and directed snap. This will generate a ton of force with minimal overall effort and will send the blade deep into even very hard wood. It's much more difficult to describe than to demonstrate in person, but I hope that made sense!
Have fun with your new Ontario! I know a lot of folks here like to customize them to their hearts content. Welcome to the wonderful world of machetes.
