Srinivas:
I took a look at
Curtain Wall DW Label 17 (I only have the AC17 Int Library) and in that label the size is controlled by the
AC_MarkerSize parameter, which is hidden (there is some other code to size the marker according to its interior text). So you might look at the copy you made, find this parameter, unhide it (by deselecting the "X" in the Parameter List), save the label, then see if this corrects the issue.
Your signature shows AC12, but the screen shots show AC15 and AC16 labels, so indicating the correct AC version would be very helpful.
David