It's possible to resize a visual and make it less than 40 px by dragging an edge of a visual. The height or width then have a red border. In the following screenshot, Height is red:
While it's possible to do it by using drag and drop, it's not possible to enter precise numbers in the text boxes to make it 30 px tall, for example. Using mouse is not precise, so I end up having 28, 31, 32 and not exactly 30 as I want.
Since it really is possible to make visuals smaller than 40 px, that must be allowed in the Format pane too. Therefore, this is a bug.