Motorola :: Class Instance Sizes In Android

I'm currently implementing some image processing code for Android. I'm aware of the memory limits and am happy to code within them. However, I cannot find any documentation that lets me work out how many bytes are used for each instance of a given class that I might want to instantiate (on the heap). I'm an experienced C++ programmer and so am relatively competent at working out such struct/class sizing issues for my C++ code (taking into account processor data path width, platform alignment issues etc). I know that Java in general is at a higher level of abstraction and so that I may not be able to guarantee particular memory usage for a general Java VM. However given that android is running on a different VM, and given that developers are strongly memory constrained: I'm assuming that there may be a relatively deterministic set of rules for working out how big a given object instance will be, given knowledge of the members.