Classifications of creatures depends on their size, each class has three types III being the smallest and I being the biggest.
Gargantuan I- 50,001+ meters
Gargantuan II- 25,001-50,000 meters
Gargantuan III- 20,001-25,000 meters
Behemoth I- 11,001-20,000 meters
Behemoth II- 9,001-11,000 meters
Behemoth III- 7,001-9,000 meters
Leviathan I- 5,001-7,000 meters
Leviathan II- 3,001-5,000 meters
Leviathan III- 1,001-3,000 meters
Giant I- 901-1,000 meters
Giant II- 801-900 meters
Giant III- 601-800 meters
Kaiju I- 501-600 meters
Kaiju II- 401-500 meters
Kaiju III- 301-400 meters
Monster I- 35-300 meters
Monster II- 31-34 meters
Monster III- 27-30 meters
Large I- 23-26 meters
Large II- 19-22 meters
Large III- 15-19 meters
Medium I- 11-15 meters
Medium II- 7-10 meters
Medium III- 2-6 meters
Small I- 0.3-1 meters
Small II- 2 cm-0.2 meters
Small III- 1 mm-1 cm