See also: Weapons
The size of a weapon compared to your size determines whether the weapon is Light, One-Handed, Two-Handed, or Too Large for you to use.
Light[]
If the weapon's size is smaller than your size (a Human using a Small weapon such as a Blaster Pistol), then the weapon is Light. Light weapons can be used while Grappling, and Light Melee Weapons can be used with the Weapon Finesse feat.
One-Handed[]
If the weapon's size category is the same as your size (a Human using a Medium weapon such as a Heavy Blaster Pistol), then the weapon is One-Handed.
Two-Handed[]
If the weapon's size category is one step larger than your size (a Human using a Large weapon, such as a Light Repeating Blaster), then the weapon requires two hands to use (and sometimes a special mount such as a Tripod).
Too Large to Use[]
If the weapon's size category is two or more steps larger than your size (an Ewok using a Large weapon, such as a Blaster Cannon), the weapon is Too Large to Use. The exceptions to this are Vehicle Weapons, which are housed in a unit that aids in their use.