Chad - using ArcGIS Pro, I opened my Catalog Pane, clicked on Portal and finally the Cloud. From there, I searched for "zip code" population and the first result USA Zip Code Areas (2017) I selected and added to a new map view. In the map view, I searched for a zipcode, zoomed to it, click a Pop-Up and in the attributes there's a field Pop17_SqMi that might work for your purposes.