According to forrestv on github:
I wouldn't recommend for you to change that value. Depending on where the problem is, it could either work or split you off from the P2Pool network.
...which I think is what happened to me & I had to re-download a new sharechain. If it works for you, that's great, but I'll take forrestv's advice & keep it at the default setting of 750000 until/if the problem has been fixed.
Sorry, but IMHO forrest was referring to tampering with the p2pool settings, namely
max_remembered_txs_size = 2500000
- not with the bitcoind blockmaxsize.
I didn't touch a thing inside p2pool, so my shares stay perfectly valid

Of course blockmaxsize is just a workaround to use v0.12 with p2pool at all. It's no solution for an increased blocksize > 1M, which we seem to get sooner or not.