Keeping this in consideration, is it width by length or length by width?
Dimensions are expressed as width by length by height or depth in three-dimensional space.
Just so, what comes first length or width or height?
It needs to be written Length X Width X Height. That is standard for measurements. It makes no difference in the order you have them listed.
Standard corrugated boxes are measured as: Length x Width x Height. (LxWxH) where the height is the vertical dimension of the box when the opening is facing upwards.