Back story (not essential to the answer)
Humans invade a forested planet and encounter the local intelligent lifeform. These creatures are tiny compared to humans and they see people as huge monsters. They want to drive the monsters away.
The tiny aliens have field artillery that look similar to those of WWII on Earth only much, much smaller.
I shall scale the aliens according to the size of their weapons, but how small can I make the weapons and still be deadly to humans?
If the aliens' field guns were about the size of an average human handgun, then that would be deadly (if the aim was good) but how small of a bore could a handgun (or alien artillery) have and still be deadly?