Skyrim's map is 101x73 cells big. 7,373 cells total.
Oblivion's map is 117x94 cells big. 10,998 cells total.
Now to count the playable cells...
Skyrim: http://img715.imageshack.us/img715/2858/skyrimmap.png
Oblivion: http://www.baldurdash.org/TESOblivion/UOPCellAudit.jpg
Neither one of those maps are the least bit accurate.
The oblivion one is the size of the in game map which doesn't show the entire worldspace.
The actual specs are:
Tamriel Worldspace Specifications:
Width...............134 Cells
Hieght..............129 cells
Total Cells.........13396
That takes into account the huge chunk missing in the lower left corner.
If you want to count Cyrodiil's usable cells use my cell grid map.
http://www.tesnexus.com/downloads/images/23939-1-1240453369.jpg
Chances are very good that the Skyrim map doesn't show the entire worldspace either. It does show the # of playable cells though. However we have no idea how many game units across those cells are so we still won't have any idea how big it actually is.