Salkkuman: I would vote for 6.5 for all sizes. For those who think komi should be larger or smaller for 13x13 or 9x9, consider this:

Yes, there are typically fewer points in 9x9 or 13x13...
...but the first move is also that much more important.

I believe most sources agree these two factors balance out so that komi should be the same on small boards.

In actuality, I would prefer a configurable komi, so we could specify it at game start, but I don't know how that would work with the current rating system. Fancier go sites are able to make games rated all the time, regardless of settings tweaks, but BK is limited in that regard. Perhaps only rated for default settings (komi 6.5, no handi)?