I don't think there is a markdown implementation of it yet but you can put the HTML in the markdown to change it.
e.g. change:
![](:/633783444f3744f49136be0b5c84d276)
to
<img src=":/633783444f3744f49136be0b5c84d276" width="400">
You can set the width only and the height will scale automatically (or vice versa).
You should be able to open the note in an text editor and do regex find & replace to change them.
Otherwise you are probably looking at resizing the images themselves in bulk.

Out of interest did you import your ENEX as markdown? I wonder if importing as HTML preseves the size that is set on EverNote.