In general, you should not worry about exactly how much space is used by fields and objects in Java. It isn't fixed and the language doesn't let you find out (well, maybe there's something in 1.5...?).
A big reason for Java's popularity is that it gets you away from worrying about low-level stuff like how the data is actually stored in memory.
Occasionally, you do care, because you are working with huge amounts of data in memory, and want to avoid using more than you have to. A little experimentation is required, then, but remember that any results will be specific to the JVM and platform you're testing on.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.