As mentioned by most here:
1. Yes, the size of the knot should equate to the style of collar.
2. Yes, the bottom of the tie should just touch your belt buckle or kilt waist, so height does factor into the issue.
However, and interestingly, not mentioned yet, the size and shape of your face, head and general physique also come into play. The size of the knot should be consistent with the size of the man.
Small and/or slim guys benefit more from smaller knots (i.e. four-in-hand) and narrower ties.
Larger knots tend to compliment broader faces, bigger bodies and/or bearded men
Part of it of course is Fashion vs. style. Fashion being what's "in" at the moment. Whereas Style transcends whim and adheres to general rules of dressing to suit your body type, skin coloring, etc.
Trust your instincts! Or if that fails, follow the advice of a trusted friend, fellow X'er or loved one.