Either doesn't really matter.
By measuring it
With measuring tape.
length times width
The measurement of length, width, and height finds the volume of a cube or rectangular prism.
When measuring a curtain, the length usually comes first. This refers to the vertical measurement from the top to the bottom. The width is then measured horizontally from side to side.
Either doesn't really matter.
Width is a length or distance. The unit of length is the meter.
By measuring it
With measuring tape.
Multiply length times width.
length times width
Those measurements are forms of spatial measurements.
The measurement of length, width, and height finds the volume of a cube or rectangular prism.
Length x width x depth.
Metric
By using measuring instruments.