The active area of a drawing tablet is the rectangle region of the tablet's surface that detects the EMR pen. Some drawing tablet manufacturers (Wacom & XP-pen) use the term active area while others (Huion) use the term working area.
When we talk about the "size" of the drawing tablet, we are referring to the active area.
The active area is usually marked in some way on the surface. Sometimes it is marked at its four corners. Some tablets show a grid of of dots.
The active area of a pen display is very easy to detect because it is the exact the same area of the display panel.
Usually when we discuss the size of an active area, we talk about its diagonal length. Drawing tablets vary quite a bit in their active area size. More here: Active area size.
The relationship between the width and height of the active area is its aspect ratio. More here: Active area aspect ratio.
