Answered
UI & Layout Composer vs. Individual Product Settings
To clarify, the stage size I set in the Individual Product Settings (or fancy product builder) takes precedence over the stage size set in the UI & Layout Composer.
So, if I have a fancy product that references a particular UI created in the Layout Composer, but I also specify a different stage size in the product builder, the stage is set by the size I entered in the fancy product builder.
That is correct, yes.