The bar width is the width of the smallest bar code bar in millimeter.
This isn't set automatically, the default is 0.5 mm. Which seems to be fine for barcodes printed on DIN-A4 paper.
This indirectly sets the dimensions of the barcode itself - the width of the smallest barcode automatically determines the width of the larger ones as well as the whitespace width.
You can also set the bar height (at least for most types which do allow to set the bar width).
So if you want to create a large barcode printed on a poster having 0.5 as the width would result in a tiny barcode.
Therefore, you would have to enlarge the value here (a value between 0.1 and 10 mm is allowed though, but 10 mm is quite large).
The same is true if the barcode takes up to much space, you would then have to reduce the value.
The problem here is the printing solution, that if this is too small, the barcode bars can melt together.
So basically the user can control the size of the barcode with this option. What is reasonable depends on the output format, how the barcode should be processed etc.
For the usual barcode scanner being used on DIN-A4, you would never want to change that setting.