It is a small continent all to itself, riven from the rest of Morrowind by the remains of a colossal crater. On a clear day (an exceedingly rare event), the peak can be seen from Almalexia, 250 miles to the south.
Meaning Tamriel overall is 675,390 sq miles. That's 1,749,260 sq kilometers. That is around the size of Libia.
Cyrodiil = 137,382 sq mi
Elsweyr = 42,577 sq mi
Valenwood = 46,713 sq mi
Black Marsh = 75,327 sq mi
Morrowind = 122,173 sq mi
Solstheim = 5,581 sq mi
Skyrim = 78,950 sq mi
Summerset Isles = 34,435 sq mi
High Rock = 45,626 sq mi
Hammerfell = 86,626 sq mi