We own the small, medium and the large versions of the Prolite pillow. My son uses the small, me the medium and my husband the large.
The medium is 18×14" and weighs in at a hefty 9 ounces. I have learned to not roll it up, but to just stuff it in my pack. The filling is the leftover punchouts of Prolite pads, so they bend well when scrunched in.
The small size weighs in at 7 ounces, and is plenty big, bigger than most any "backpacking" pillow you will find.
The large is the size of a pillow at home, and is worth the weight if you need neck support.
While the series of pillows are not truly lightweight, they are worth the weight for how they do not get lumpy, flat or slide around. They come back to life within a half hour and are very thick. It has allowed me to sleep on my side.
I have had mine since 2004, and I haven't left it behind since then.