Code 39 ExtendedLink

I'm trying to print CustomerID in BarCode form using Code 39 Extended. At time, the barcode will be printed, but at time it just simply show the word "size"!?
The problem is that if you use several characters (e. g. small letters) that are not included in Code 39 but in Code 39 extended, the character is made of two symbols in the barcode, which can increase the effective width of the barcode. You should increase the width of the barcode in the way, so the maximum code length your require will fit into the control to avoid problems.