How to get used to drawing human head smaller?

That's an unrealistic standard, leaves no room for caricature or the ability to draw different 

That's an unrealistic standard, leaves no room for caricature or the ability to draw different 

It's not the usual method, but it can be useful. Some artists divide the body by measuring chest size. Another way to determine how tall a character will be is by measuring the distance from the pectoral to the collarbone.

Pd: I'm not good at English. Sorry if it's not clear :c

I'm curious if you do any sketching first or any other preliminary groundwork, like laying out the initial shapes, posing, and composition? Or do you go straight into the drawing? If you can catch the proportion issues in the groundwork/sketch phases then you can make any necessary adjustments before committing it to the final linework.

practice. keep practicing drawing a smaller head on a body structure until its something you do naturally

Not to sound off-putting or anything but erm...short people irl do exist, so if you draw characters or people who are not "7 or 8 heads" tall, it's completely fine. You shouldn't try to "be perfect" in Art anyway, cuz perfectionism isn't even real - it's very subjective.

Personally for me as an Artist for the last 22 yrs (and counting), one of the things I've discovered is that many of these "art rules" that people say "you should/ must follow" to "draw good", all seem to be based on colonial (white) beauty standards, (which let's be honest - it's not realistic, nor very reflective of the real world/ real people and doesn't cater to other cultures who exist), so yeah - if you draw short figures, I would say to embrace, rather than to be upset or frustrated by it, cuz you could be representing a group of people in media, (via your Art), who never really got proper representation before, anywhere else. If someone tells you that your figures look too small or short, just remind them that in many countries around the world, there are people who are that size and that just cuz they're not accustomed to seeing them, doesn't mean that they don't matter or don't deserve attention too. And if they still reject you cuz your view is "too progressive", feel free to walk away from that person.

But of course if you really wanna fit into the mold, (like most people usually do), then the most I can say is to do more bio studies, look at other people's work and keep practicing, (since I can't see your work looks like, to know how I can assist with corrections if required.)

Map out the whole body's proportions before adding defining features. Dont draw the head and the face and the hair, *then* add the body. Just do a rough mannequin- my head start as a circle for the top of the skull, and a rectangle that's kinda bent like a piece of paper to form the plane of the face. Super basic shapes. Then add in your ribcage rectangle/sphereoid, hip rectangle/sphereoid, shoulder circles, and limbs. I tend to start limbs as just sticks then add mass to them with oblong almost-rectangles. 

Honestly mapping out the whole body and post before adding mass and character appearance is the one thing that has helped my proportions the most, and given me the most freedom to change things. I used to hate when I'd draw the face great then the body was too big/small and I didn't want to erase anything to fix it. So mapping things out first helps me not feel bad about erasing to fix things, and helps me catch those mistakes early.