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.