OVERVIEW:
The barcode area is delimited by the selection rectangle shown when the barcode is selected. "Design Mode" needs to be enabled to be able to select a barcode and resize it. This selection rectangle delimits the space which the ActiveX object occupies inside the Excel document. That means it could overlap other objects depending of if its layout/size.

RESIZING ACTIVEX BARCODES
!!! important note !!! To be scannable, a barcode needs a reasonable white space around it. If the selection rectangle is sized smaller than the barcode drawn, the barcode will be cut inside the selection rectangle and not scan.

You can resize the selection rectangle of a barcode as long as all the bars of the barcode are still displayed.

INSTRUCTIONS FOR ENABLING SELECTION AND RESIZING IN OFFICE 2007
1. Click the Office button (upper left corner) then excel options in the popular tab "check" the "Show Developer tab in the Ribbon" and click OK.
2. Select the "Developer" ribbon and click on the "Design Mode" to enable activeX barcode control selection.
3. Click on the barcode that is overlapping other areas. A rectangle selection box shows around it.
4. Grab one of the selection/resizing handles  of the selection rectangle and move it so that the Activex barcode object resizes so that will better fit the size of the excel cell.

WHY IT SHOWS RIGHT WHEN BARCODEMAKER IS NOT INSTALLED
When a document containing ActiveX objects is opened on a PC with no BarcodeMaker installed, the metafile drawing of the barcode is drawn and no ActiveX objects are created since the PC doesn't have the ActiveX barcode control. So there is no overlapping issues since there is no ActiveX control overlapping other areas of the Excel document.