Changing the Size or Position

For Size, see here.

For Position, see here.

Change the Data or Options

Double-click the barcode to bring up the dialog. Double-clicking on any non-OnMerge barcodes will bring up Word's usual dialog.

Alternately, you can right-click on an OnMerge Barcode and select the OnMerge Barcodes icon from the pop-up context menu.