How i can make that a polygon fill the 80% of the width of the screen?

Question

I have a simple square (polygon), and i want that it fills the 80% of the width of the screen

In other words, i want to position the square on the center on the center o

Answer 1

Use a projection that allows you to address the viewport in terms of ratios. Like this:

glMatrixMode(GL_PROJECTION);
glLoadIdentity()
glOrtho(0, 1, 0, 1, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

Now your OpenGL coordinates map the range [0,1]² to your screen. A 80% width would be 0.8 in the x coordinate.