The largest muscle size you can ever achieve is determined in combination by your genetic predisposition and your workout pattern, consistency, effort, and to a lesser degree nutrition. Beyond that you can push closer to your limit by using anabolic drugs like testosterone and it’s derivatives, Growth Hormone, IGF-1, Insulin and others.
However, even those individuals who use any or all of these drugs will eventually reach a limit as well.
The human system (any biological system for that matter) can only adapt to a finite and specific amount of stress until it eventually breaks down.
This is a well known phenomenon and is an example of hormesis (not to be confused with homeostasis)
Hormesis is the concept that a given stressor (such as exercise) can force the system to adapt in a positive manner but the amount of stress that compared to the benefit/reaction is not a linear relationship but rather a curved relationship.
In other words, there is an amount of exercise that is too little and an amount that is too much and an amount that is just right.
The ‘just right’ zone is where you want to be to stimulate muscle growth, and as you get stronger this zone changes.
You need to do more work and more often to continue to grow over time. This however reaches a limit at which point the entire system breaks down (over training).
Most people don’t do enough work to ever reach a true state of over training so this really isn’t an issue.
The key to stimulating maximum muscle growth is pushing the upper edge of the ‘just right zone’ without ever going over it.