Yes, of course on some locations a bigger kite is needed, agree.
But firstly, the title was almost a general question "what is the biggest kite that makes sense to use for a SURFBOARD".
Not about the biggest kite / board that makes sense.
And IMO it does not make sense with bigger than 10 m2 on a surfboard, if you can ride in all conditions.
Secondly, I assume John has Irish roots, and his avatar says West Sussex (South coast of UK), which is the same waves and wind patterns as here in DK and Sweden t.ex.
Thus it is low pressure systems, with lots of different wind from 0 to 40 knots, and NOT steady low winds for a long time and clean swell like in other spots.
Also, the waves are only partly swell, but most often wind generated - so makes even less sense with a 12 or 15 m2 on a waveboard (apart from just getting OUT of course, but there are much better boards for this, and at least a very different question)
Still say without doubt, keep your 12 but sell the 15
Peter