Weapon sizes is a mechanic in MADNESS: Project Nexus, which dictates what weapons can and cannot be one handed depending on the character's strength stat. This can be calculated with the equation of: 15(Size-1.7). For example, if a weapon has a size of 3, it would require a strength stat of 20 to wield with one hand.
Certain attachments onto firearms can increase the size of the weapon. For example, the MAC-11 has a default size of 2, but its default magazine adds 0.25, making the size 2.25. This would require a strength stat of 9 to be able to one hand.
Size 2 = Strength of 5
Size 3 = Strength of 20
Size 4 = Strength of 35
Size 5 = Strength of 50
Certain weapons are impossible for normal characters to use with one hand, such as the Dragonslayer, due to the size being 12. The Dragonslayer also has the trait of "TwoHands" in the game's files, making it impossible for non MAG Agent characters to one hand it.