I got an answer on LF forum:

My trays have radiused bottom corners
The flat bottom size (before the radius begins) is 15 x 18"
Halfway up into the radius, the size is 15.5 x 18.5"
At the top of the radius (1/2" off of the bottom) the size is 16x19"
The sides taper out to 16.5 x 19.5" at the rim.

They are top-notch quality, I love them when printing batches of 11x14's...