Assign Hit Dice; Hit Dice by Size

Source: SRD / Basic Rules

Hit Dice by Size

Monster Size	Hit Die	Average HP per Die
Tiny	d4	2½
Small	d6	3½
Medium	d8	4½
Large	d10	5½
Huge	d12	6½
Gargantuan	d20	10½
^hit-dice-by-size