Try this:
.hikashop_products_listing input.hikashop_product_quantity_field {
width: 50px;
text-align: center;
}
Note that I added the center alignment... keep it or leave it...
To eventually make the +/- better accessible, use this:
.hikashop_products_listing a.hikashop_product_quantity_field_change {
font-size: 24px;
padding: 0 10px;
}
Thanks to the class "hikashop_products_listing", all of this applies to product listings only, not to the product pages. If you want it there, too, just leave that class away.
As far as I can tell, you're not using any HikaShop CSS file, so add it to your template's custom.css file.