Imagine a planet, 3/4 water and 1/4 landmass. No axis tilt so the climate is moderate throughout the year. IE peak temperatures at 100°F at equator.
It orbits a sun about the size of ours and has a slightly different orbit.
What is the smallest circumference I can make my planet be and still have ocean currents supported by ice caps?
Its okay if I have to change the orbit, and assume that whatever the size the magnetic field will still protect the planet.