Unfortunately due to the way the back office search is coded searching a barcode will only take you to a product, not individual sizes etc. You could raise this on the Ideas Forum for consideration in a future implementation of the product.
A potential alternative solution (depending on what reason you had for searching) could be to have ran a Product Extract. You could then Ctrl + F / find within that CSV file using the barcode, which would show you the specific size and various other details.