How to take a photo that shows the real proportions?

Specifically, can it be done with a mobile? And at what distance and height should the camera be?

As long as you're at a good distance from the mirror if you're taking a reflective pic and holding the camera/phone around your waist at a perfectly parallel angle to your body.

Other peps should really chime in

As a general rule follow what Alu just said. 

What I found interesting with the mock-ups I had done at a doctor's office, is that my legs started looking way too long compared to my body even after only 8cm.  It took me almost a year to realize the reason behind this..... the optical illusion came from the fact that my original picture fit exactly from top to bottom  in a standard sheet of paper, but my mock-ups were having the top of my head and forehead cut-off to fit into the same size sheet because of the longer legs...... so my mock-ups had longer legs with shorter upper portion of the rest of my body(head)....... I noticed this same phenomenon when people post pictures of themselves to ask for opinions, and their entire head is erased or blacked out, and surely it looks like a lot of these people have naturally long legs.