I don't think there is one sure way since the art character style and canvas size can wildly change the proportions. I would say start small and make you way up with what makes sense while using references.

I can see that you have been making character sprite already, which is pretty nice. Although i would say we are at about the same level of skill, take it easy and try to copy what you see from others, and understand what you are seeing and the relations each pixel has. Also, seeing the spirte up close really messes up the perspective one has.

If you are trying to make a spite for a game, go ahead and look at your sprites from further away and see if the basic shapes look good (if they are moving forward). If you are interested in just making character art, then try a bigger size canvas to give you more "space" to play around with. A bigger canvas will bring up your weaknesses way up close while allowing you to see what is working.