Determines the number of bytes that would be required to store this with store.
If this has a fixed-sized type then this function always returned that fixed size.
In the case that this is already in serialised form or the size has already been calculated (ie: this function has been called before) then this function is O(1). Otherwise, the size is calculated, an operation which is approximately O(n) in the number of values involved.
a Variant instance
the serialised size of this