After copying a new created banner in Animate2021 I changed the canvas (document) size, adjusted the elements, changed the dimensions of some elements and tested the html output. Some of the assets got the wrong (old?) size in the export. Some were correct.
Then I clicked the assets in animate until I got to the deepest level (tween, groups, etc). I did not change the properties of the elms. Some of the assets were displayed correctly - some not. I had to import the wrong displayed elements (svg-files) again, to get a correct output.
All elements were shown correct in the authoring environment (just playing the timeline by hitting enter).