In practically 99% of the cases, precise size selection is important but not critical when it comes to refrigeration pressure regulators. Just determine maximum normal refrigeration load, desired pressure drop (frequently 2 psi or 0.14 bar), and nominal evaporator temperature. Enter the regulator manufacturer’s selection tables and choose the closest — not the next larger — regulator port size.
In some cases, the available regulator connections may not be the same size. However, if the system pipe size is materially different, say more than two sizes, the regulator and system size calculations should be rechecked.