How do I calculate the size of pump I need for my system

Water pump? Heat pump?

It's a water circulating pump for a 12 radiator ststem.

Many grundfoss pumps have 3 speeds and will work fine for only 12 baseboard radiators.

Most boilers have a recommendation for min flow rate through boiler, some provide specific pump makes. After the boiler needs have been met, a pump to circulate through the radiators is not as crucial most pumps will be more than adaquate. Be careful with the temp of the return side of the boiler if it gets to low and your boiler is prone to corrosion it could condensate and prematurely rot your boiler.