You can do it without using the camera. Scroll the motion bar to the start of the animation (after you've set up the animation). Now zoom as needed while keeping the Isometric view. Finally right click on the key for "Orientation and Camera views" and select replace key. This will zoom the model at the start.
Now if you want model to fit to screen while it is exploding, got to end of the animation, set the desired zoom and add a key over there.