In general,
NPT threads are sized so that they can be cut onto the ends of NPS (Nominal Pipe Size) pipe. Thus, 3 NPT threads fit 3 NPS pipe, 1/4 NPT threads fit 1/4 NPS pipe, and so on. These threads are tapered so that male and female parts will wedge together, creating a relatively large sealing surface compared to the size of the parts.
It's important to realize that the Nominal Pipe Size has almost nothing to do with an
actual dimension of the pipe (especially in the small sizes). The OD of 1/8 NPS pipe is actually .405 OD.
The orifice size of your solenoid valve is an internal characteristic that is closely related to the valve's Cv.