How large are the planets in Bioware's Star Wars MMORPG? How do they compare to the continents in World of Warcraft?
Since TOR is unreleased yet, no one can honestly answer this question. However with everything Bioware has shown so far, TOR appears to mimic or improve (hopefully) upon WoW in many aspects, such as companion characters to speed up preparing raid parties.
Furthermore in Star Wars Galaxies, the planets where quite large and offering plenty of exploration (if that is what you are looking for). Bioware is also focused on story, so I doubt a planet will be superficially large with large expanses of empty ground for size sake.
In terms of total land mass, my gut feeling is TOR will be comparable, if not large, than WoW. Don't forget that TOR will include space combat, therefore space zones must be included too.