The size of packages is not stored in the API unfortunately. You'd likely need to grab the sizes with a script that mounts the DP and gets the size with du for every item in the "Packages" directory or something. Its going to be a slow script if you have an even moderate amount of packages.
Just use "ls -l /Volumes/YOURCASPERSHARE/Packages" in our environment at least it appears to return pretty useful results in one of the columns and it's very fast. du seems to go wandering off inside the pkg and dmg files which is probably way more info than you need.
@Kedgar A package that's too big can cause problems and delay delivery of the package's contents. Packages that are smaller than expected may indicate something is wrong with how they were built. If the package you were given is all you have then make the most of it!
