I recently did KOA-DEN in row 9 (economy plus). I'm a one-bag traveler and usually toss my backpack in the overhead to preserve leg room. On this flight after boarding I would need to do some work so I decided to try my bag under the seat. My bag for this flight was a ULA dragonfly which fits into the "personal" item sizer easily. While the underseat area on seats A, B, C are all wider than they are on say a 737... there was a severe height restriction there I didn't anticipate. It seems on the 757-200s there is a big plastic box that holds the life preserver... which made the height of the underseat space ~6 inches. Quite annoying.

Thought I'd post this here for anyone hoping to store their bag under the seat in front of them on this aircraft.