HGd10PHB - Size Categories

Size Categories
Size categories are somewhat fluid as many things have irregular shapes. The following represents a general guideline based on typical objects or creatures. You can find a list of metric prefixes here. An sM is a solar mass equal to the mass of the Terran Sun.

In certain scenarios, usually combat-related, differences in size require modifiers. Strength is generally built into the creature's stats. However, larger objects are easier to hit. For every size category greater than the attacker, the defender gets a -5 to defense rolls. For every size category less than the attacker, the defender gets a +5 to defense rolls.