Making the ruler line a different color is really smart.

Nice work, and such a cool pick! Something that helps me when learning a new subject is measuring proportions of a mass/shape against nearby ones--like in this instance, the distance between the head and the start of the arms seems to be the same length as the cranium mass. I think even sectioning the body into different parts based on thickness, like the neck, torso, and tail, could also be worth a try. Of course, since we are sketching from imagination, you are more than free to change proportions and play around with different breakdowns, so this is one way of approaching it that could make it easier to tackle.