Well, it's not just the number of feet, but also the temperature that you want to be able to operate at. Heat transfer in fluids is a logarithmic function, so the closer the storage tank water gets to the temp of the hot tub water, the less heat transfer will occur. The larger the heat exchange surface, the better the transfer. In other words, it never hurts to have a larger heat exchanger.
Sizing for immersed coils is never easy, because the coil shape, restrictions to flow, and other factors can all change it dramatically. I'd say that the simplest thing would just be to buy a 50-foot roll, since it tends to be sold that way. 1" would transfer more than 3/4", but not bend as tight, so it would depend upon where you wanted to install the coil. My thought was a coil around the entire tub, against the wall, near the bottom (but a few inches up). I would going to construct benches to go over it, with slats underneath to protect feet from accidental contact with the coil. Nicely spread out like that, the transfer will be better than if it is all bunched in a tight coil, replacing the woodstove and using the "fence" to keep folks away. Of course, it will require more carpentry to protect feet from the hot coil. On a large coil like that, 1" would be fine.
Joe