Just the CPU and no GPU? If it's just the CPU then a good 240/280mm rad will have plenty of headroom. The reservoir can be as small or as big as you like. To be honest though, it's going to cost a lot of money for a custom watercooling setup to cool just the cpu and I don't think I would do it just for that. When you consider how much it costs for a good rad, pump/res, CPU block, tubing, fans and coolant then adding a GPU block at the same time isn't that much more expensive. You would need a 360/420mm rad though, maybe more depending on the GPU.
A AIO or very good air cooler will work out much cheaper than custom water for just cooling the CPU.