Why bother with a dictionary on rotations since you could opt for a handbook that would be much more useful (e.g. Ferri's Care of the Medical Patient for med or Surgical Recall for surg)? You could also opt for a PDA with the software of your choice, including a dictionary if you wish.
If you must have a dictionary, Stedman's makes a softcover, abridged dictionary, but it may not fit in a standard pocket.