Don't know how else you can get the file size unless you locate a plug in (several allow you to interact with files) that will list file sizes for you.
I suppose you could use a script that uses the file reference to temporarily insert a copy of the file into a different (likely global) container field in order to compute the file size...
I used your second option. Exported the PDF and re-imported it into another container field and grabbed the file size, then copied that into another field and finally deleted the imported PDF. A bit long winded but it works,
The grabbed file size retrieved is diferent from when you click on the actual PDF and Get Info, why aren't they the same?