Weapons come in four different sizes. These sizes affect what size of creature can use this weapon and how
Considered a light, one-handed weapon for all creatures of Small size or larger
Considered a one-handed weapon for Small creatures such as halflings and gnomes
Considered a two-handed weapon for Small creatures such as halflings and gnomes. Medium-size creatures can wield Medium-size weapons with one hand.
Small creatures, such as gnomes and halflings, cannot wield Large weapons. Humans and other Medium-size creatures can wield them with two-hands. Larger creatures, such as giants, can wield Large weapons with one hand.
If you wield a second weapon in your offhand you can get one extra attack per round with that weapon. You will receive a penalty of –6 to your main hand attack rolls and a penalty of –10 to your off-hand attack rolls. There are several ways to reduce these penalties. Additionally there is the Improved Two-Weapon Fighting feat, which grants a second off-hand attack, with an additional –5 attack penalty.
Using a double weapon is the same as using two weapons (consider the off-hand as wielding a light weapon).
Off-hand Weapon Damage
Any weapon used in the off-hand only receives one half your normal Strength bonus for an attack.
Two-Handed Weapon Damage
Any weapon being used in two hands delivers one and a half times your normal Strength bonus.