There are four types of creature sizes in IB2 (for encounters; on adventure map there are just normal sized creatues):
1. Normal (1x1 square on enocunter map, use a 100x100 .png, extend the y-size of the .png by 100 for each attack frame you want; standard with one additional attack frame is therefore 100x200 png)
2. Wide (2x1 squares on enocunter map, use a 200x100 .png, extend the y-size of the .png by 100 for each attack frame you want; standard with one additional attack frame is therefore 200x200 png)
3. Tall (1x2 squares on enocunter map, use a 100x200 .png, extend the y-size of the .png by 200 for each attack frame you want; standard with one additional attack frame is therefore 100x400 png)
4. Large (2x2 squares on enocunter map, use a 200x200 .png, extend the y-size of the .png by 200 for each attack frame you want; standard with one additional attack frame is therefore 200x400 png)
You can set the creature size under properties of the creatures (creatureSize) in the toolset: 1 is normal, 2 is wide, 3 is tall and 4 is large.
Wide and tall creatures need testing, any feedback welcome.