I have finished entering the details needed to publish an extension, including uploading the VSIX file and reviewing the additional fields populated from info within the VSIX, into the marketplace form, and uploading a 128 x 128 image.
The problem is I don't completely understand how the requested inputs will translate to the exact appearance of the overview title/tiles you see here: https://marketplace.visualstudio.com/.
I've searched without luck for some way to preview this, before publishing, without luck. (One reason I want to preview is that, when I pull a snapshot of the marketplace screen into Illustrator the dimensions and positioning of images and text relative to one another don't seem to mesh with requested dimensions of the image upload. I.E., most images actually seem to be more like 38x38, whereas a 128x128 area encompasses the image and a product title, which is separately entered.) Am assuming it's "sloppy" to officially publish something that you then need to fuss with or delete and republish.
Mads did a build presentation that covered some of this a few years back on Channel 9:
In the discussion section of that link, he pointed out that the sizes are 175x175 and 90x90, were optimized for the website rather than the extensions dialog, which no longer does the preview.