I am considering setting my next D&D campaign on an earthlike but small planet. By "small," I am imagining a world that still feels vast, yet is circumnavigable over land and/or water with low or no magic, though probably with significant difficulty. I was thinking perhaps 1/6 earth radius, though I'm making that number up. Physical properties would be assumed to be earthlike (probably), due to greater planet density, and there would likely be a diversity of weather phenomena around the planet. As for geography, I am imagining a single large continent, as well as an archipelago of islands covering a large portion of the planet.
For further size context, I imagine that military conquest on the scale of Alexander the Great's campaigns, or maybe the Mongol conquest of Russia, should be able to seize the entire world. The setting is a traditional middle-ages style D&D world, with magic existent but not abundantly used.
Without significant magic, how likely is it that a landlocked city would realize that the planet is spherical and small?
EDIT: Naval technology is high middle ages, around 9th century. The continent is essentially a very large island, so sailing is necessary in order to circumnavigate.