By default the image will show full-size, or responsive to the topic width if it is larger. For most cases, that's preferable.
Recently (as of the last couple years, in any case), NodeBB will explicitly set the image size so content isn't shifted downwards as images are loaded, but there's still no functionality for a user to explicitly denote image size.
The reason why that is is simply that it is not part of the Markdown spec. It is possible to add, but it is not trivial, because you have to think about how you want to define that markdown code so it is readable to end-users too 😄