sizeof in C# just used for valuetypes in unsafe mode(in version 2 no neeed for unsafe mode)
I don't know what could be good for this?
you can try GC.GetTotalMemory
Method after and befor creating your object,(but you should know other threads may run concurrently and that method doesn't give you a proper result)
I myself tried something now
private double size()
simple s=new simple();
it gives me 16 bytes if I consider just one integer field it gives me 12 bytes that 4 bytes was for an integer field(the sizeof(int) is 4 bytes)