As we can read in the Construction-Set's help-file:
"Each exterior cell is 8192 units by 8192 units or 385 feet by 385 feet."
The world map can display a square from -29/+29 (upper left) to +25/-25
(lower right) when you include cell 0/0, you'll get 55*55 cells
Vvardenfell spans from -16 to +20 in X-direction
and from -16 to +23 in Y-direction that's about 37*40 cells
(with still a lot of water included)
Sorry, have to calculate in metric values
385 feet = 117 meters
54*117 = 6.32 kilometers (3.92 miles)
37*117 = 4.33 km (2.69 miles)
40*117 = 4.68 km (2.91 miles)
Morrowind's World-Map can display 6.32*6.32 = 40 square-kilometers,
and that's 15.4 square-miles unless my conversion metric<->imperial is wrong.
Vvardenfell itself is about 20 square-kilometers (7.8 square-miles)
So "Oblivion's size" as described by gamesas, will be about as large
as Morrowind's World-Map, and about twice the size of Vvardenfell.