There is currently no size limit op_return your node won't consider a valid block. Currently it is only enforced by not being passed in mempool. Anyone can go straight to a sympathetic miner and drop 100Mb op_returns into every block. That means there is no such thing as a "full block" Want proof? Go look at Loops timeline and see that one of the first things they did was push an op_return 4x the current "limit" then brag about it.