The PortalPy module is still around but it is being superseded by the ArcGIS Python API. I don't believe what you ask is doable directly or only through PortalPy, but you can do it using the new ArcGIS Python API:
>>> import arcgis
>>>
>>> url =
>>> username =
>>> password =
>>> item_name =
>>>
>>> gis = arcgis.gis.GIS(url, username, password)
>>> item = gis.content.search(item_name)[0]
>>> item_full = gis.content.get(item.id)
>>> item_full.size
3850776
>>>
Personally, I got a bit of heartburn over how items are returned when using search in the new API, but that is a soapbox for another venue and another day.
If you don't or can't utilize the ArcGIS Python API, the guts of the code from the Example: Copy content can be used to write a script for accessing an item's size.