I need to size a centrifugel pump for a system I'm installing.
Can you please tell me a simple way to size the pump?

All the pump manufactures list the information on there web site. You need to do your calculations and find a pump on there web site that will work. There is NO simple way to size a pump. Many items have to be taken into consideration length of pipe,size of pipe,number of elbos,system pressure,liquid being used as a transferr agent,height of water colume,temperature,type of pump materials,and environmental temps. I am sure I have missed a few but as you can see it is not an easy job.

Most heating contractors just replace the pump with the same style that is already in the system or the boiler manufacturer includes the pump with the boiler. Since you are building a system I recommend you obtain the services of an engineer. Many pump companys offer a service to help in this area if you are going to use there pumps.

Call R.L. Deppmann Co. (248) 354-3710, and ask them for help. I used to work there and we helped with sizing all the time.