I don't think the coil is in any trouble of overheating as long as you keep the water circulating. Your coil material is an excellent choice, I doubt it would fail unless it was in continuous contact with the burning coal for a long period of time. Don't worry about the expansion as it will be pretty even throughout the coil. It won't grow that much anyway. The location isn't real critical as its pretty hot anywhere inside the stoves firebox.
One thing I would do is check your boilers HW coil, especially if the legs are in and out at the top. Old coils, especially with well water, can be plugged with sediment or scale which would reduce their efficiency severely. There are some potent (flourosilic acid? leave this stuff to the pros) and other, more safer things available at the plumbing house to clean you HW coil.
I would think bigger is better on the tank size, 60 minimum and 80 is better (teenage girls? +).
I also would look into backup power, @ minimum for your stove coil pump if you are in a place like I am. Losing power in the winter for 2-3 day clips is no fun (well, now it is. but the neighbors hate you
). I prefer a whole house solution in this area, and reccomend 6,000 watts or better.