What size boiler for a 3000 square ft house

A big one… but not too big!

How many square feet of wall is the heat going to be migrating thru? How many square feet of glass is the heat going to be migrating thru? How many square feet of ceiling is the heat going to be migrating thru? How many square feet of cold floor is the heat going to be migrating thru? Are there any skylights? What kind of insulation is in those walls? Ceilings? Floors? How many floors, ranch two story modern, split level, three story attached town house?

I have calculated over 60 BTUh Per sq ft of floor space and have also calculated under 25 BTUh per sq ft of floor space. It all depends on how well it was constructed.  In 1950, homes had minimum insulation standards and were built fairly energy efficient.  but not as well as they are built today.

3,000 [] x 25 BTU/[] = 75,000 BTU's

That's my rule-of-thumb calculation and that would get me in the ballpark, but before I commit, I would pen and paper a room-by-room heatloss total. You want to feel some level of confidence when you order your boiler, no?

Of course, that rule-of-thumb multiplier would slide up or down depending on the condition of the house.

The G215/4 has a net BTU rating of 149,000. That's 49.6 BTU per square foot. Almost double what @Alan (California Radiant) Forbesquoted. Are you near the coast or high elevation? Very poor insulation? That's a lot of BTU's.

"Where are you getting 140?"

right here, the net rating