Looking at your square footage if you want to keep the pole barn really warm you might need a G400, if the garage and pole barn are kept cooler a G200 might do it.
A lot of guys around here in VA do blowers in outbuildings and bump the temp back to save wood, then bump it up when working in them. Radiant will need to be kept a steady temp.
Still curious what tonnage the heat pumps are. The model # should have a # divisible by 12.
Oops, you did post the heat pump tonnage. I missed it, sorry.
Sizing of the boiler would depend a lot on how warm you intend to keep the pole barn and what burn times you are expecting. A G or GS 200 would be about perfect for the house and garage. 5.5 tons is not a heavy load on a G200. The pole barn could get burn times pretty short on a G200 though if it is kept at 70+ F.
How often you would be near the unit to throw on a few sticks of wood would factor in too.
I'm comparing Heatmaster models as that is what I am familiar with, hopefully that gives you a reference point as you browse the options.