The answer is to the custom casing dimensions is...
When casings are desired to be different widths (in ArchiCAD 20 that I know of), gang the side of the window you want to mull to another window and check the casing box right beneath the box you checked to gang that side of the window. Then, to get that casing to meet the other windows casing you are mulling it to (which will be ganged on the appropriate side of course), adjust the shim space under "Nominal Sizes and Tolerance". The casing width with adjust according to the shim space number.
... and that is how you do custom casing dimensions in ArchiCAD 20.
It took me around a year to finally realize this answer! I am going to enjoy drawing windows so much more!
This is much better than using the Morph tool to fill in the gaps for every piece of the window.
>ArchiCAD 23, 7000 USA FULL
>iMac, 27-inch, 2020, Processor: 3.8 GHz 8-Core Intel Core i7, Memory: 64 GB 2667 MHz DDR4, Graphics: AMD Radeon Pro 5500 XT 8 GB