Basic Rules/Creature Size

Normally creatures of Small or Medium size have a standard reach of 5 feet. For creatures above medium or below small their reach tends to be as is listed in the creature size chart. Listed within this chart are “Tall” and “Long” creatures, this is generally a distinction between creatures that stand upright, or creatures closer to the ground like a wolf, with these general body shapes determining the rate at which their reach progresses as they progress through the sizes.

Creatures Smaller Than Small
Creatures below the size category of small take up less than 1 square of space; allowing more than one such creature to fit into a single square. The general rules is creatures of this size category can occupy 4 of the space that a single creature one size category above them can occupy. As such 4 Tiny creatures can fit into a square, while 16 diminutive creatures can fit within a square, and 64 fine sized creature can fit into a single square. This value doubles if the creatures are capable of flight as they can then occupy the vertical space of the square as well, changing the progression to 8 Tiny, 32 Diminutive, or 128 Fine sized creatures. Usually fine sized creatures within gatherings of this size instead of being tracked separately are instead treated as a swarm.

Creatures that take up less than 1 square of space typically have a natural reach of 0 feet, meaning they cannot reach into adjacent squares, requiring them to enter an opponent's square if they wish to make melee attacks. A creature can always attack into their own square allowing them to attack such creatures normally. A creature with a reach of 0 threatens no squares around them and cannot flank an enemy.

Creatures of Tiny or smaller sized wielding a reach weapon are instead treated as 1 size category larger for the purposes of determining their reach (up to Small). Spells or abilities that improve a creature’s reach (but not size) simply allows creatures of this size to be treated as a size category larger (up to a maximum of Small) for determining reach for every 5 feet of additional reach they receive. Once a creature of this size is treated a small for determining reach they only gain additional 5 feet of reach for every 10 feet of reach an effect would grant beyond this. If an effect would cause these creatures to become small size or larger they calculate their reach and improvements to their reach as normal.

Large and Larger Creatures
Creatures above the size category of medium take up more than 1 square.

Creatures that take up more than 1 square fall into one of two categories of reach progression as “tall” or “long”. Tall creatures (generally humanoid shaped or similar) typically have a natural reach of 10 feet or more, meaning that they can reach targets even if they aren’t in adjacent squares. Long creatures (generally quadrupeds like wolves) only have a natural reach of 5 feet when large and increase to 10 or more at higher size categories. Most creature types and even some spells that modify form may specify or change your type of reach progression.

As the improvement to their reach is granted by their size and not a weapon property, they may attack and threaten all squares within their natural reach, making it difficult to approach such creatures as most medium sized creatures must pass through a square of a creature’s reach before they can get adjacent to it to attack.

When a creature above medium uses a reach weapon you double their natural reach as normal for determining what squares they can attack and threaten with the weapon; but cannot attack or threaten squares within their natural reach unless they change their grip as normal for a reach weapon.

Square Occupied by Creature Two Sizes Larger or Smaller
A creature can move through a square occupied by a creature at least two size categories larger than it without needing to use acrobatics to move through their space. Doing so provokes attacks of opportunity as normal for movement.

Attacking Larger or Smaller Creatures
Creatures larger or smaller than medium gain a bonus/penalty to AC and Attack rolls and CMB/CMD based on their size category, as seen on the creature size chart. This bonus/penalty applies when attacking and defending from other creatures, but does not apply when doing aid another checks or when attacking an object (such as hitting an AC 5 to hit a specific square).

Creature Size chart
All creatures above or below medium receive the following bonuses and penalties based on their size. Bonuses/Penalties to AC and Attack do not apply to MSB or Saves against manuevers and rather have their own separate bonus/penalty to be applied instead.