This is currently a limitation of ArcGIS Online due to the way ArcGIS Online hosted feature services are stored. There have been enhancements logged for this:
NIM093828 : Expose geometry fields (length, area, etc.) for read-only display in feature services.
ENH-000088963 : Provide a method to automatically calculate Shape fields (Length and Area) in ArcGIS Online.
The workaround is to create a field before publishing and calculate the desired geometry (length, area, etc).