Both games use a construction set scale of 128 units = 6 feet, or equivalently, 70 units = 1 meter, and a time scale of 30:1.
In Oblivion, the playable range is just over 5 km x 6 km (at the CS scale), or at 30:1 scale, 152.5 x 181.5 km.
Westernmost landmark: Garlas Malatar (3.132 km W, 0.190 km S)
Easternmost landmark: an Oblivion Gate north of Sundercliff Watch (2.886 km E, 0.131 km S)
Northernmost landmark: Serpent's Trail (0.558 km E, 2.457 km N)
Southernmost landmark: Deepscorn Hollow (1.730 km E, 2.627 km S)
In Morrowind, including Solstheim, it's slightly smaller: just over 5.5 km x 4.8 km (at the same scale), or scaled, 165.6 x 145.5 km
Westernmost landmark: Stahlman's Gorge (Solstheim) (3.136 km W, 3.106 km N)
Easternmost landmark: Omaren Tomb (Azura's Coast) (2.383 km E, 0.542 km N)
Northernmost landmark: Caverns of Karstaag (Solstheim) (2.779 km W, 3.211 km N)
Southernmost landmark: Arano Tomb (Azura's Coast) (1.765 km E, 1.638 km S)
Map coordinates were obtained from the Google maps at UESP.