if the class was had a SerializableAttribute and its object was serialized with BinaryFormatter, the serialized size is equal to the size it occupied in memory?
Edit: Additional question: is there a better way to find how much space an object occupies in the memory? is this method approximate enough?
No. The members of a class/structure is stored in memory with padding to align them on an even word boundary when needed (according to respective data type). The serialised data has no such padding.