I may have misunderstood the question. Are you trying to figure out the peristent size, or the in-memory size? They are not the same. The persistent size of an object is much larger than the memory it consumes.
My previous post assumes you are interested in the persistent size. If you want to know the in memory size it is not quite that simple, but here is an article that I found that will allow you to figure it out.
http://www.webgain.com/communities/developers/resources/articles/newsletter_1.html Cheers