I think the appropriate komi on non-standard board sizes (except for very small boards) will likely remain debatable. It would take a lot of data from strong players to even approach settling the issue.
Even for 9x9, there can be lively debate about the “correct” value of komi:
Perhaps, even the komi for 19x19 is debatable. Komi is a relatively modern invention and has changed within recent times. For example, the Nihon Ki-in (Japan Go Association) changed komi from 5.5 to 6.5 (for Japanese rules) in Sept 2002.
If you can coordinate with your opponent beforehand (rather than just issuing an open challenge with a fixed komi), perhaps you could try bidding or the pie method:
- In the pie method, one player selects the komi and the other gets to select whether to play black or white.
- In the bidding method, each players proposes the komi and the player bidding the largest amount gets to play black, while that komi is used. A coin flip could settle ties (but at least the players agree on komi).
By the way, the correct komi for 5x5 is given as 24 or 25 (territory vs area scoring) since black should be able to prevent white from making a living group.