I have thirteen radiators so I need to know what size pump I need. Not all the radiators are getting hot. Some staying cold.

Pretty difficult to tell you when we have no idea as to the capacity, climate, insulation etc. Is your boiler adequate-first thing to check? Bled all the system? Could be corrosion in the piping is restricting the flow. One floor or many as that influences design pressure? Suggest a good local HVAC person.

I seriously doubt the pump is the problem. We are talking about a hot water boiler, are we not?