Preview: image width in units other than px are not converted
The image widths don't make sense to me. They are displayed much to small. In addition, sometimes the tag is not compiled but displayed. See screenshot.

{width=…} is probably handled as an HTML attribute—as such it would always be in pixels. The generated preview is always in px width, regardless of what length unit one specifies.
One exception is using %, which seems to work rather well. Using mm as an example would only work with CSS property values (and if the output media is known). Using absolute length values isn’t recommended, although I’d like that too.
I’m unsure if this is a Panwriter preview or a Pandoc problem, though.
EDIT: Just checked. The preview generates an <img … width="100mm"> when it instead should generate <img … style="width:100mm">. A HTML parser will typically regard the former as a width of 100 px.
Yes, pretty sure the attribute is currently just passed through as is in the preview, unlike pandoc. This would have to be implemented in https://github.com/mb21/markdown-it-pandoc/