Scaling 3d model to fit screen size

I have built a slideshow that loads images, video, and 3d models. My problem is making the 3d model scale and center automatically when loaded and on screen resize. I can get it to load properly scaled and centered, but I keep having issues when the screen resizes. Can anyone help me scale the 3d model so that no matter what it is always filling the screen and centered whether the screen is in portrait or landscape?