The Sea of Rhûn, according to Tolkien's maps, covers roughly 400 square miles. The Celduin (River Running) flows from the northwest into a northwestern arm of the sea, while several unnamed rivers flow from the northeastern shore. West of the Sea of Rhûn is the land of Dorwinion and roughly 200 miles to the South is the eastern end of the Ered Lithui.
The Sea is a remnant of the Inland Sea of Helcar, which extended all the way to the Wild Wood in the First Age, but all the eastern lands were changed in the battle of the Valar with Morgoth.