The line should be determined by an economic pipe sizing program.That will make the fluid velocity in the pipe stay within certain boundaries (not too slow or not too fast). If the flow is too slow, the pipe is probably larger than necessary which is more expensive to purchase and install. If the pipe is too small, the high fluid velocity will be noisy and probable erode the piping system.

The control valve needs to be sized using a pressure drop that is about equal to the system pressure drop (pipe friction + filters + heat exchangers ++). If the valve is sized with an arbitrary pressure drop of 10 pounds, it is a crap shoot as to it's ability to control. If the valve pressure drop is too low, the system will control the fluid flow, not the valve. If the pressure drop is too high, the valve may be much smaller than the pipe, noisy and probably wear out in a relatively short period of time. Also, the small valve, relative to the pipe size, may be under a great deal of stress when the pipe expands and contracts due to temperature changes.

So, in my vast experience as a valve sizer, they are usually smaller than the pipe. In order to get the valve to fit in a larger size pipe, you need 2 pipe reducers, one on each side.

If the valve and pipe are the same size, something is wrong. If the valve is larger than the pipe, everything is wrong and somebody spent a significant amount of money for a large valve and 2 reducers.