Dimensions mean always inside dimensions in mm on the following way:
Length (L) x Breadth (B) x Height (H)
Length (L) = the longer dimension by the opened box
Breadth (B) = the smaller dimension by the opened box
Height (H) = the size between the opening surface and the bottom of the box
By boxes with two components there is a fourth dimension, which is the high of the top segment.
For example: 355 (L) x 205 (B) x 120 (H) / 40 (h) mm
Sizes of a sheet
The sizes of a sheet are always given in mm - the breadth x the length of a sheet (or the board), where the length means the parallel direction to the track of the waves.