When someone says any "X" mil, they are referring to the additional stroke. If it's a 4 mil that means 4 mm more stroke over stock. And so on for 5, 7, 10, etc.
Stock Rod length is 110mm. Typically when you hear long rod thrown out as a term, they will mean 115mm rod. The theory is the longer rod puts less stress on the piston and have a longer dwell time. There will be different pistons to run a longer rod.
421 will typically be 4mil crank, 68mm bore.