How to control the size and view port of the model reading from a model file?

i find the size and view port of the model reading from a model file very difficult to handle and control.Is there any way to make it easy? And how to set the parameter of OpenGL?if you know,please tell me.

Thank you very much!

It doesn’t seem like you are explaining yourself very well. What viewport are you talking about? If it is the rendering viewport, you can easily use glViewport for it. It has nothing to do with models.