Skyrim's heightmap is rectangular and uses 119 x 94 = 11186 in-game "cells". The engine uses the same cell size as in Oblivion, Fallout 3 and Fallout: New Vegas - 57.6 metres (63 yards) to the side, 3317.76 m² (3 969 square yards) of area. The full map thus has an area of about 37.1 km² (14.3 square miles). Around a quarter of this is not playable, stuck behind invisible borders.
The playable area is roughly the same as the one in Morrowind and Oblivion and less than one thousandths of Daggerfall's size.
In addition, the game features a good part of the surrounding area of Tamriel as low-quality "fake" terrain meshes.
For comparison, the heightmaps of Skyrim (upper left corner), Morrowind (upper right corner) and Oblivion (lower left corner) look as follows, to scale (courtesy of Lightwave from Bethesda's forums).
Most of Oblivion's heightmap is not playable, while most of Skyrim's and all of Morrowind's map can be visited in game.