Default to d8 for custom monsters—it offers a balanced chance of hitting, missing, and critting. Any die size is fine as long as overall damage per round stays consistent. Here are some thematic guidelines:
- d4: Undead (slow, with BIG bonus damage).
- d6: Goblins (small, chaotic, likely to miss or crit).
- d8: Humans (balanced and reliable attackers).
- d10: Beasts (stronger than humans).
- d12: Giants (superhumanly strong/accurate).
- d20: The mightiest creatures (massive damage).