Scott7m, what I was trying to elude about the pump was, by going with large pipe you are able to effectively reduce some of the head pressure associated with a smaller PEX pipe; YES you will still have some head pressure associated with any elevation changes.
Also, it does matter what your BTU load is because if you undersize your feedline, your building will not warm up cause it not able to transfer enough energy from the boiler (water jacket) to the building.
In my current installation the smallest feed line I run is 1", cause I go from the boiler (open system) thru a Heat exchanger and out to the garage (4000 sf). When I build the house, I plan on running 1 1/4", so that I add some Snowmelt, THROW AWAY THE SHOVEL AND BUY AN EXTRA SPLITTING MAUL.