Whoa...hold on there...Sizing the water to air heat exchanger takes in some factors that are sometimes variable.
You need to know three biggies...water temp delivered, water flow rate through it and the c.f.m. of air across the coil. Don't get even one of these wrong or you'll be crying in your beer.
And further more, you may actually need to make the duct/plenum larger
around the heat exchanger than the existing duct to get your required
size H/X installed.
And remember...if you're also making lots of DHW on a 0* degree day then the water temp delivered to the coil may not be what was figured and that's not the day to find that out.
Next to the boiler...the H/X is very critical to home heat comfort.