Correct piping

Bob Chauvot

Hello, I'm replacing my old Bryant with a new Crown Bermuda BSI276. I have a two pipe system and the piping on the new system is similar to the old. As you can see in the photos, The old Bryant had 3" risers and header, leading to 2" mains. My new Crown has 2" risers coming from the boiler. In the piping schematic Crown suggests the header be 3". My questions are: Do I in fact need to have the header 3" and if I do, do I increase it to 3" at the top of the risers or do I need to keep it strictly on the horizontal header? Thanks ahead of time for your input.
Bob