query on size of opengl view

I am using a view in an MFC dialog, the view takes up about a 1/4 of full size window. I wanted to find out the width and height of the view and tested it out using input values in dialog to draw a polygon. Is the size always fixed at around width-800, height-500? Then i draw a triangle and found that using different vertexes values, the width becomes different. Can someone explain?