The official rules don't directly answer your question. However, the Dungeon Master's Guide p. 249 illustrates the relationship between squares and hexes when it comes to creature size:
For cube spells, I suggest using the same relationship between squares and hexes for three reasons:
- There exist no other official guidelines regarding hexes.
- It is proportional to the size of creatures (which take up smaller areas on hexes than on squares).
- It is simple and unambiguous to use, i.e.:
| Cube size | Hexes | Hint |
|---|---|---|
| 5-foot cube | 1 | |
| 10-foot cube | 3 | 1 ring around an intersection |
| 15-foot cube | 7 | 1 ring around a single hexagontile |
| 20-foot cube | 12 | 2 rings around an intersection |
| 30-foot cube | 27 | 3 rings around an intersection |
| 40-foot cube | 48 | 4 rings around an intersection |
| x0-foot cube | 3 ⋅ x2 | x rings around an intersection |