As a rectangular region, we can discuss the size of the active area in different ways
Width & Height
Diagonal length
Area
But most often we will talk about active area sizes using the diagonal length.
If you need help picking the right size for your tablet, consult choosing the right tablet size.
You can often see the diagonal length in the names of pen displays. For example:
Wacom Cintiq Pro 27
Huion Kamvas Pro 16 Plus 4K
For pen tablets, manufacturers often use descriptive names such as "small", "large" instead of numbers.
Wacom Intuos Pro Medium
Wacom Intuos Pro Large
By looking at the Wacom's product line and naming, we can establish a decent framework understanding how descriptive words like "small" and "large" map to numbers. Note that the size names are different for pen displays and pen tablets.
If we look at how drawing tablet models are distributed along this scale we'll see that there is a lot of variation.
Is it reasonable to use the active area's Diagonal length vs the Active area's area measurement.
I think it is, because the diagonal is reasonably well correlated with area for drawing tablets.
Let's see how diagonal and compare buy plotting the values for 180 different tablets.
For pen tablets:
And for pen displays:
Overall, I think using the diagonal is reasonable.