I want to buy this radiator which has 1/4" fittings for the hose so thats a hose with a inside diameter of 6mm. isn't this small to get low temps? or will it be fine? if not what kind of screw do i need to be able to attach a bigger hose in diameter?
I think you should consider a high end AIR cooler if you need to ask this question.
But the Answer is that is the "Thread" or connecting screw. It will have a second measurement for the ID or inter-diameter of the hose it supports, and if you are using a compression fitting it will have an OD or Outer-diameter of the hose.
I really really suggest you read up a lot more about liquid cooling before you buy anything or risk doing some major harm to your computer.
Not saying don't do it. JUST Please make sure you know what you are doing first.