I guess you could make an empirical assessment of the amount of memory occupied by a certain number of objects, by reading in a large number of objects multiple times and tracking memory usage via a profiler. However, whether you could trust the results to be reliable on other hardware, OS's, or JVMs, I can't say. It may be a case of 'suck it and see'.

It is easier to measure something than to understand what you have measured...
Anon.