It's larger than a mass market but smaller than a trade.  Is it trade or mass market?  It is 8 x 6 inches.  What is this size called?

8X6 is commonly referred to as a trade sized paperback. There isn't really a certain size related to a "trade" paperback, its just commonly used for the larger paperbacks now. Most of my trade size are right around that 8X6.

There is that one other size (I forget the name tho), they are the same width as mmpb but an inch or so taller.  Publishers thought they would be a hot ticket, but didn't happen. 

No point in splitting hairs, anything larger than a mmpb is considered Trade size.  Unfortunately, this catagory is not standardized so they can be anywhere from just a bit bigger to much bigger.  I have a couple that are in excess of 8 X 11.

Perfect Paperback.

Also seen them called "Premium" paperbacks.  And one name that was a play on the words Easy Read.  (Eaze Read?)

FWIW, technically "mass market" and "trade" are not designations of size, but whether the book is "strippable".  That is, trade paperbacks that don't sell must be returned intact to the publisher for credit.  For mass market books, the bookstore need return only the cover--and it is in the contract that they must destroy the rest, which is why coverless books are actually stolen property.  See the copyright page in any mass market paperback printed in the last decade or so for details.  (An 'S' in a triangle on the back means "strippable".)

In practice, of course, people use "mass market" to mean small paperbacks, and trade to mean larger ones.  However, there are many small ones that are not technically mass market (usually from small press publishers or gaming companies), and I have seen larger ones that are (because they are strippable).

Lately several publishers have started coming out with taller mass market paperbacks (at a higher price, of course :-( ), which do nothing except cause shelving problems for most people.  Certainly the print is not larger (which it often is in trade paperbacks).