If you have nothing better to do, take a consecutive portion of your recent scored 19x19 games and, in the final position of each game, find the single largest rectangle you can fit into your own area – that’s area, not territory, meaning that you can include your own living stones (and any captures).
Catalogue the area of the maximal rectangles and average them – this gives you the “rectangle measure”.
I don’t know yet whether the measure is at all meaningful, which is part of the fun.
Here is my result, from ten of my consecutive scored games at mid-SDK.
So my rectangular measure is 43.2, or 44 to the nearest rectangle.
If I discard Koba 78 as an outlier then I get a measure of 39⅓, rounding to a rectangular 39.