All these guns are assumed to be made for approximately human-sized users. Someone who is abnormally larger or smaller would modify the damage of their guns accordintly.
Generally, someone who is twice as tall and six points stronger can use a weapon that does two extra points of damage. All other characteristics of the weapon remain unchanged - concealability is relative to size, after all - but the weapon is extra diffcult to make and costs an Item schtick. Assuming a base Strength requirement of 3, this works out to Strength 9 for +2 damage, Strength 15 for +4 damage, and Strength 21 for +6 damage - characters of such size and strength generally prefer melee.
The formula is +2 damage per 6 Strength, half rounded up, but first subtracting one point. Do not forget the size requirement.
Someone who is half as large would lose two points of gun damage. Ignore any Strength requirement in this case. As an alternative, they can use a standard gun in two hands as though its concealability was once class worse.