I understand the block limit is an anti spam measure,
but why can't miners just form their own consensus of
how big a block is acceptable? If more than 51% of the
mining power agrees a block is too big, they will ignore
it and build a longer chain.
Ask Mike Hearn why he thinks checkpoints might be necessary, just in case this happens.

I fail to see what Mike Hearn or checkpoints has to do with this conversation. Please stay on topic.
It may have been a mere quip, but it was on topic.

Are you suggesting that the threshold for consensus in a hard fork be lowered to 51%?
Not necessarily. It depends what kind of change are we talking about.
What I'm hypothesizing in this thread here, is that maybe blocksize
shouldn't be a protocol rule to begin with.