[Turtle91]

While I tend to agree, Diap, I can think of a couple reasons why you'd want an image like that. e.g. a large map in the main screen with a list of locations below. A tap on the location would display a pop-up with an image of that location zoomed in - rather than just enlarging the original image.

Also, the OP's question was: "... how do you adjust the size of the pop-up window and get rid of the scroll bars?"
So adjusting the window size and having an image fill the smaller window could be useful, depending on what the OP wanted to do with it.

Now that Marvin 3 supports ePub3, I'll have to start playing around with some of these functions to see what I like!