In the vanilla Minecraft animation method, the frames are always perfect squares and the size is determined by the width of the animation strip. There's no way to specify anything else.
You may be thinking of Optifine's animation method which allows you to define custom areas. That's not covered in this tutorial, though, and would be off-topic. Please feel free to make your own thread asking about it in the Resource Pack Help forum.
I hope that helps you.
Thanks for your answer, I thought I heard it was possible in vanilla Minecraft too, but as it seams, it's not. Mojang Studios should it add it though.