NOTE: This question and answer is only valid for older (before version 3) OpenGL.
First of all, glFrustum is independent of your screen resolution.
This function define a volume in which the scene will be drawn, that is, all element of your screne that throw outside this volume will be ignored/cut. Coordenates for glFrustum are relative to your scene, not to your screen.
Take in consideration that for a correct representation, other parameters must be configured correctly like scene rotation/translation, etc.
glFrustum(-10,10,-10,10,10,1000); glFrustum(-100,100,-100,100,100,1000); glFrustum(-0.5,0.5,-0.5,0.5,0.5,1000);
All the above were giving the same output of the images to me on the screen
Not exactly, if your 3D model is near the POV (camara), example 2 will cut it where example 3 will not. But as you increment/reduce the "near" distance in proportion to "left", "top", "bottom" and "right", the result is similar.