Defining an OpenGL unit

glViewport actually defines the size of your window, so glViewport(0,0,600,600) defines a window of 600 x 600.

By default opengl's coordinate system sees 0,0 as the center of your window and 1,1 as the upper right hand. If you want to change that you use either glOrtho or gluPerspective. Using glOrtho to figure the unit/pixel ratio is easy:

given:

glViewport(screen_left,screen_top,width,height);
glOrtho(negative_x,positive_x,negative_y,positive_y,positive_z,negative_z);

where:

Screen coordinates

screen_left // the x coordinate that your window is created at 
// relative to your screen
screen_top // the y coordinate that your window is created at 
// relative to your screen

Opengl Coordinates

negative_x // the left x coordinate that opengl sees
positive_x // the right x coordinate that opengl sees

negative_x // the bottom y coordinate that opengl sees
positive_x // the top y coordinate that opengl sees

Be sure to note that screen coordinates are not opengl coordinates

then:

x_ratio = (positive_x - negative_x)/width;
y_ratio = (positive_y - negative_y)/height;

Try this tutorial's lesson 2 & 3 on setting up the viewport correctly.