The behavior you are observing is not a bug, but is by design. As explained in the forum thread you referenced and as you found in your application, a change in the image, or in other words - commit of the current tool, only happens after you click on a tool to invoke it. It makes no difference if the same (in this case the Resize tool) or another one is clicked, but it is a required step for the changes to take effect and it has no workaround.
Regarding the Image Size that is shown - the estimate represents the raw data. i.e.
in case each pixel is 4b, how big is the image. As the available export formats compress the image, it does not actually represent the size when you save to any of them.
What you can do in order to disable it is change the template, for example collapse the label in the tool settings. We will also consider a change in this regard for one of our future releases.
I hope this helps! Let us know if you have other questions.
All the best,