Each instance of a reference type has two fields maintained by the runtime - a method table pointer and a sync block. These are 4 bytes each on a 32-bit system making a total of 8 bytes per object overhead. Obviously the instance data for the type must be added to this to get the overall size of the object. So for example instances of the following class are 12 bytes each:

class MyInt
{
…
private int x;
}