Is there a maximum horizontal acceleration for passengers? Are there any regulations regarding the maximum acceleration forces that passengers should experience during takeoff/flight/landing? 

Or what would be considered to be an acceptable horizontal acceleration? 0.5g? 1g?

I am contemplating how to achieve very good STOL characteristics for a small  commuter aircraft while still maintaining a high wing loading.
It depends on several factors:
1) How long is the body subjected to these forces?
2) What's the posture of the body?
3) To what extent are the passengers used to g forces (for example due to training)?
... 

However, early experiments showed that untrained humans could withstand g forces as much as 20 g for less than 10 seconds, to 10 g for 1 minute, and 6 g for 10 minutes.