The paper size selection facilities usually are quite printer dependent, and the input tray selection and paper size selection mechanisms may interact in strange and mysterious ways.
letter, legal, ledger, oversize, a0, a1, ...
These are standard paper size names.
11x17, tabloid
These are usually aliases for ledger, but depending on local conditions can select different types of paper.
paper=name
The paper=name form allow users to use options such as -Zpaper=b3, which may be useful for systems that have an unusual or nonstandard input media selection mechanism.
Media Type Selection
Media Type is not the same as paper size, and corresponds to the name assigned to a particular media. Of course, the issue is complicated by the fact that some media have standard sizes as well. Again, the input tray selection, media size, and media type selection will interact in confusing and mysterious ways, depending on the whim of the printer firmware iplementors.
You will also notice that there is no general mediatype=name selection mechanism. This is due to the extremely different way that the media names must be passed for PostScript, PJL, and PCL.
plain, preprinted, letterhead, transparency, glossy, prepunched, labels
These are commonly used media type names gleaned from various PostScript Printer Description Files, Microsoft printer drivers, and arcane lore of the Printer Working Group. Note that these are not accepted terms in the paper industry for any of these type of media.