Overwatch – How Much Health Does Symmetra’s Photon Barrier Have?

According to the Symmetra In-depth Strategy Guide by FuriousPaul charging it until the circle is full should be enough to deal max damage.

When charging up a Photon Orb, it will reach its max charge before the game fires it for you. This means that you can fire fully charged orbs more often than if you were to wait for the game to fire them for you. The Orbs will reach max charge when both of the circles in your crosshair meet with each other. The animation on the right shows when it reaches full charge, but I made it so I waited until the game fired for me:

I also made the same experiences (also for Zenyattas Alternate, btw.) in live games but will try to confirm this in a custom game seeing the damage numbers and edit this answer then.

According to the [WIKI] it does protect against all stuns and knockbacks.1:

Particle Barrier: When activated, Zarya will completely surround herself with a barrier for a brief period of time, absorbing a large amount of damage and making her immune to all negative effects (such as knockback or stun) while the barrier holds. Damage dealt to the Particle Barrier contributes to her Energy.