How do I tell a "mass media" book from other paperbacks?

By Mass Media book, I think you mean Mass Market Paperback (MMP). The size is usually about 4-1/4 x 6-3/4". Trade size is usually about 5-1/4" x 8".

mass market paperbacks are the paperback size that you most commonly see in any stores.

trade size paperbacks are larger, usually the size of a hardcover book but with a paper cover instead.

There is a new paperback size that has just started being used which is about an inch taller than a regular mass market paperback, and it is priced accordingly (at $9.99 when I pick them up).

If you are referring to MMPB (Mass Market Paperback) that would be the traditional 'pocket book' size.  The other sizes have different names...it's becoming confusing, but tradesize covers several oversize paperbacks.  Then there's the taller format that Sara refers to, and (I haven't seen one yet) but a really new size that is shorter and wider (IIRC)?

and (I haven't seen one yet) but a really new size that is shorter and wider (IIRC)?

Oh yeah, I've seen that one too. Its almost square. Also, priced at $9.99. Personally, I hate it and see no reason why anybody would prefer it to a regular MMP.  It will fit awkwardly into any existing bookcase or stack of books. .... I'm hoping that nobody will buy that size.

Also, my best guess about the ones that are only an inch taller than regular MMPs .... is that the publisher can print a longish book in fewer pages, and can then use a cheaper binding to hold the thing together. (And charge more money for it, while they are at it).  Of the new tall MMPs, they seem to me to have a disproportionate number of binding issues ... I get them more from my library sales than from PBS, but often the copies I see have pages falling out, etc.

I've only seen the squarish ones at Costco.

You can't really tell on PBS, while some are listed as MMP - most are just listed as "paperback".  While on the book you can go to the "buy from Amazon link", Amazon does list the measurements.