I have a bunch of polygons in a layer file and I want to reorder those polygons so one of them is "sent to back". I am using ArcMap 10.5.
I would use the Advanced symbology tool Symbol Levels as discussed a bit here: [How to force polygons to the bottom, without compromizing drawing speed? Go to Symbology tab, Advanced, Symbol Levels. Click on the use symbol levels box, then move the polygons up and down to control drawing order. However, if you have a lots of ones needing to go back because they are larger, I would do two layers based on a definition query on size (as Vince suggested).