As far as I can see from the java.io API, you would have to create a new File() for the folder. Then sum File.length() for each file returned by File.listFiles() recursively for each sub-folder. I guess this would only give an estimate of the actual folder size.
A platform-dependent alternative that might give a more accurate figure would be to use Runtime.getRuntime().exec() to submit an appropriate command to the underlying OS. That's not as straightforward as it sounds.