What Size Trolling Motor?

How much thrust do I need to move a Tandem Island (with two aboard) at a reasonable speed? Don't need to go fast, just move along when the wind dies.

I have a 55# Newport with a 100ah LIFePo4 battery. Have it on the coast of NC and use it to get in and out of canals and narrow channels where it's not practical to sail. In moderate winds (5-8mph) at 1/2 speed moves it along at about the same speed as pedaling. Full speed a little faster. Had as many as 4 adults on board. Last weekend used it about 2 hours over about 4 miles. Meter said 70% battery remaining. So far pleased with the performance. Have not used it against strong winds or currents, believe that will push its limits.