The furnace area where I intent to insert the hydronic coil has a maximum inside dimension of 25"x25". The air channel is 22"x22".
The brazetek catalog has several dimensions for coils. There is a pdf that lists all dimensions. A 20"x20" coil will have 20"x20" coil area, and 24.5"x24.5" outside dimension, not counting the inlet/outlet pipes. This is the size that will fit. The inlet and outlet pipes will come out of the duct through pipes.
I looked for a coil that I could insert at an angle, to increase the coil area, but there is none that would fit, and it would make installation more difficult, so I will order a 20"x20" coil and install it in the furnace. The coil won't be plumbed yet, since there isn't a solar system yet to connect it to. There will be a 20"x20" paper filter installed against the coil, upstream of it. I may also install filters at the two inlet grilles. This will improve air quality and prevent the hydronic coil from clogging with dust, which would decrease its efficiency. Due to the low temperature of solar heated water, I will need the coil to keep optimal performances.