I threw a quarter in the shot so you can get a sense of the scale. Keeping them small is important to me for a couple reasons:
1. This is the step closest to the idea. Quite often drawing them is a mad dash just trying to keep up with the flow of ideas. If I keep them nice and small it allows me a quicker response to the initial idea and forces me to keep things simple - which brings me to my second point...
2. Clarity. You need to make sure that your idea is being stated clearly. Comics/films are a visual medium and if the audience can't follow with the pictures alone, you're not taking advantage of a tremendous opportunity. With comics and animation it's an incredible challenge to get the audience to care about your characters. They're drawings (or in the case of computer animation, Puppets). Your challenge is not just to get the audience to believe in your world, but to engage and care about the characters - to feel. If you've set up a confusing shot that an audience can't visually follow or is sending mixed messages, you're shooting yourself in the foot. With a small drawing there's no room for unnecessary detail and you're forced make sure the idea is stated simply and clearly.
And now back to killing monsters ;)