[Closed] How to render objects edge to edge

I have some objects in a scene that I need to render. My goal is:

1. to make a render with the dimensions 100 * 75
2. that the rendered objects boundingbox in the image is either 100 pixels wide or 75 pixels high.

I am having problems with 2). I have made a brute force method that renders unhidden objects, checks rendered image and if boundingbox is less than image size, zoom camera until no more space. This approach has some problems.

1. SLOW since it may take 30 renders to get to final render
2. I do not know how to center object 100%

Is there another way to do this which is faster and where the objects are 100% centered?