As long as you have the drive and intention, you will be able to teach successfully! There were no truly fat people though.
The all-inclusive nature of yoga makes the practice accessible to anyone who is interested.
If you want yoga for larger physiques, nothing is stopping you from building that yourself.
I have found Tai Chi to be far more superior.
And it works much better to improve flexibility for those of us who never had a yoga body to begin with.
Not much to worry about.