In which step you begin to determine the proportion of the human form?

When you finish the line of action and ready to add the ovals,will you think about the proportion of the human forms or you will determine it after you add the ovals. And in the latter case,what if the size of the ovals is not suited to the proportion?

The ovals represent head, chest and hip. They pretty much determine the proportions. If they don't, then you should practice making them fit.

The idea of practicing to start with this "ovals" is to ingrain into you, that you always nail the proportions of the figure, before you waste any time adding details. When your knowledge of anatomy grows, you will add an eyeline, a chinline, a centerline and ears to the head, make the chest "oval" into a flattened egg-like shape and the hip "oval" into something akin to a bucket-shape. You will also learn to determine "landmarks" on the body, that will help you determine the exact size and position of chest and hips.

If you want a good introductory course, I would recommend Proko.com . The free courses on the page offer plenty enough material and explanations to understand the underlying basics, paying for the premium course is more of a gesture, that you appreciated Stan's work. It adds a bit extra, but that's more a "thank you for your support" from Stan, than introducing any new concepts, that aren't covered by the free course.

First you need to identify as if the position was a line, how it will be, is a kind of stick man, but then will add more ways to improve the structure of the body, add can form round to the chest or head or curves, etc. After that, you can add a little more detail to the basic shape of the body. I hope to answer your question :)

When I draw the head, though sometimes I need to resize it since it often ends up being too small.
Don't worry too much about proportion in the gesture stage, your focus should be on the force/movement behind the pose, and getting caught up in proportional problems early on is a pretty easy way to end up with a stiff pose

I determine the proportions from the very first stage. I think first I try to create a pose (which I am having trouble with) and then I think about the proportions. I create a pose very lightly, I outline it with a few strokes, draw a very simple sketch. So, I do the composition of the figure, its scale and pose, and then work with the proportions.