Title: Clear all records of field
You may use Clear method for all records of specified field. For manipulating of records use First, Edit, Post and Next methods.
procedure TForm1.Button1Click(Sender: TObject);
begin
with Table1 do
begin
First;
while not EOF do
begin
Edit;
FieldByName('SIZE').Clear;
Post;
Next;
end;
end;
end;