This method does work but unfortunately doesn't work like you would expect or how I had in mind when I designed it. The reason for the current behavior is because of how the internals of Fusion 360 work. Basically, this method is only effective the very first time your add-in run. It will set the size of the dialog. However, if the user manipulates the dialog and changes the size, Fusion 360 now remembers this new size as the default and it overrides whatever you set with setDialogInitialSize. What's needed is a setDialogSize where you specify and have full control over the size. Some additional capabilities needed to be added to Fusion 360 core before we could add that to the API and it hasn't happened yet.
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Brian EkinsInventor and Fusion 360 API Expert
Website/Blog:
https://EkinsSolutions.com