SizeOf function is described below:
Returns the number of bytes occupied by a variable or type.
So you can know a variable's size through the function. It is different from Length function which aim is to return the length of a variable.
Here is an instances for SizeOf function, you can refer to it.
type
CustRec = record
Name: string[30];
Phone: string[14];
end;
var
P: ^CustRec;
begin
GetMem(P, SizeOf(CustRec));
Canvas.TextOut(10, 10, 'The size of the record is ' + IntToStr(SizeOf(CustRec)));
FreeMem (P, SizeOf(CustRec));
end;