If you are using a profile arg in your shortcut to start AutoCAD then you will likely need to re-export the profile after you resize the dialog.
Try this:
Run the script telling you what size the dialog is...
Open AutoCAD by double clicking on a drawing, not by using the shortcut...
In that drawing, open the mtext editor and resize it to the size you want.
With AutoCAD open run, the script and see if the size has changed. It will probably be the same (provided it is the same profile)
Close AutoCAD, and run the script once more, it should now show the dialog size as being different.
Open AutoCAD once again with a double click on a drawing, not using the shortcut try the mtext editor and report what size it is...
If it is the same as when you re-adjusted it last, the problem lies with a profile that is being loaded at runtime overwriting the registry values. If it back to the small size again, then it might be something else loading at runtime that is updating it.