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Does this mean I have to adjust the width and height in the export area in Inkscape?
I guess that depends on size of the image. What size is your image? And what is your "desired size"?
If so, do we have to adjust the width and height of the selection or page or drawing or custom?
It depends on how you have set up your document and how you are using Inkscape. Here's what I would suggest.
Decide what size you want your image to be. It sounds like it can either be 86 mm wide or 180 mm wide. And no more than 200 mm height. If it's not that size, then scale it to that size. Just scale it on the canvas, not using Export PNG dialog.
Then use Document Properties > Page tab > Page Size > Custom Size > Resize page to content. I would not set more than 1 or 2 px margin, if any. This makes the page size exactly the size of your image.
By doing that, you can use the Page option in the Export PNG dialog. And you don't have to bother with changing the Dpi or adjusting the size in the Export PNG dialog. By the way, the Dpi should be set to 96.0, if you're using version 0.91.x or higher. For earlier versions, use dpi = 90.0.