I wonder what the miners think about the blocksize proposal. I believe they can choose the size of the block to be anywhere between 0 to 1MB today. Should the 1MB limit be raised, the final decision will remain with the miners, right?
Correct. Miners do not have to include any transactions in their blocks if they so choose.
Also, miners do not have to build off any particular block. Lets say a miner starts issuing blocks filled to the brim with 1-satoshi transactions. The other miners could all agree (or collude, if you see it that way) to reject the 'spam' block and build off the previous one.