I am encountering problems trying to resize the default popup feature. Regardless of my approach, The popup will not properly size the window to my photo and description. I understand this may be impossible currently, according to previous threads:
...But it is my job to check.
The only mildly odd bit about this data is that it was converted from KML to GeoJSON (not shapefile) to accommodate very long descriptions, longer than the dBase-Shapefile limit allows. This should be irrelevant.
Currently, this isn't possible with pop ups hosted with ArcGIS Online. We are working to improve this experience in the future but it would be helpful if you posted your specific needs on the ideas site Auto-size/Customize width of ArcGIS.com map popups.