New hydro air system

So a little background. Heating a 220 year old 2800 sqft colonial in New England. I am currently heating with a hardy h4 boiler built in 1991. Current set up in the house is a lay down oil furnace circa 1985. I have a water to air hx in the plenum and have burned very little oil since buying the home 4 years ago. I have found that whenever I have tried to use the furnace it takes a long time for it to heat up the water in the hx before it actually can send some decently hot air out of the vents. I average around 12 cords Of hardwood from November 1 to around May 1.

Now to the new System. I would like to try and cut down on my wood usage and would like an efficient back up system in the house that I can go away for the weekend and it will keep the owb warm and heat the house. I have purchased a heatmaster g200 to install this fall. It should help cut down on my wood usage. Second thing is that I have purchased a 10 year old Weil mclain gold series 185k btu oil boiler to replace the old oil furnace. I plan on removing the old oil furnace and installing the boiler in its place. I will tie the supply and return lines for the forced hot air together with a plenum that has the water to air hx and the blower in it. I plan to run the supply line from the owb through a flat plate hx on the boiler then to the water air hx then back to the boiler.

Now a couple questions. First is what size flat plate hx do you guys think I need?
Second question is about the proposed set up, any suggestions on it? Thanks!

I have not had a heat loss calc done for the home. the old furnace was a 200k btu oil furnace. I have done some improvements such as attic insulation, wall insulation and air sealing. I have done my own manual j calc before and while I think I may be a little oversized I don't think I am way out of the ball park. Yes I plan on using the oil boiler to act as a hydro air system. The owb will pump 24/7 the loop of water. It will go from the owb to the oil boiler through the FP then to the 24x24 hx in the air handler then back to the owb. When the owb isn't running the oil boiler will heat the owb (195 gallons) which should act as a buffer tank to avoid short cycling of the oil furnace.

You are correct. The only thing in the loop on the oil boiler would be the flat plate. The loop would simply be pumped(taco 007 would be fine I think) from the oil boiler, through the flat plate and then back into the oil boiler. This would keep the oil boiler at temperature except if the owb runs out of wood or I go away for a weekend. When the temp dropped down in the oil boiler the aquastat would kick it on and it would heat the water in the opposite direction. Instead of the OWB loop being heated by the OWB, the oil boiler would heat the loop which would for one keep the owb from freezing up and it would be able to heat the house because the loop goes through the water to air hx in the plenum and that would still work just as if the owb is still running. Did I clear it up at all? I am still wondering about sizing my Flat plate, I think the a 40 would do it, a 50 for sure but any suggestions on that are welcomed.

I certainly don't dislike your approach with just running the water to the flat plate and back out to owb. I did think though that the owb would simply act like a buffer tank for the oil boiler keeping it from short cycling. I have homemade spray foam lines with about 3 degree loss in the 200ft loop.