I could be wrong, but I don't believe Fonts can be stretched to distort the aspect ratio.
What exactly do you mean that you have "grown the font size as big as possible"? In most cases, you can type in a font size larger than the max size in the drop-down. Even though Excel allows a max font size of 72, you can type in larger sizes than this, like 150.
On the other hand, this also makes the font very wide, also... so perhaps you want to alter the aspect ratio. Here is something to consider, it won;t work with standard fonts, but for barcodes, it is pretty good.
Duplicate the text in two cells, one above the other other (use =a1, or equivalent). Then set the Upper cell to have a vertical alignment of Top, and teh lower cell to have a vertical aligmnet of Bottom. Next, reduce the height of both rows, until the "tex" merges, top to bottom at the cell boundary. You will probably want to play with this, to prevent too much overlap... you also need to turn off your grid-lines. Ultimately, you should be able to double the height of the barcode without making it any wider...