This is an image of what an unabridged dictionary looks like:
These kind of dictionaries are large and heavy enough to be used to break down medieval city walls. They can't be held in one hand -- they can barely be lifted with two. You normally find the print editions of these only in libraries and other institutions that house large reference tomes, and usually kept on special lecterns to make them easier to find and peruse.
With regard to your question, the "desk" in "desk dictionary" is misleading. You might think it implies:
Something which fits naturally on a normal sized-desk
when actually the meaning is more like:
something the average person might keep on their desk.
In other words a "desk dictionary" is of a size that is convenient to keep around your home. It's necessarily abridged to keep it relatively small, such as this, my Webster's New Collegiate Dictionary from 1979 (and which I probably haven't opened for 20 years):
Small enough to pick up with one hand; small enough to keep conveniently on a desktop or bookshelf. Of course, nowadays my cell phone has access to complete, unabridged dictionaries, so this print edition is just a memento.