Dimensions can be useful to determine shippingcosts or exclude a product from free shipping. If you correctly set the dimensions, this can help you to calculate the actual costs and pass this data to your shipping company to get an accurate indication of the costs.
If this does not apply to your company you can dismiss the dimension notifications.If you do want /need to set the dimensions and weight, first make sure all the correct metrics are set. You can find these settings in WordPress admin panel > WooCommerce > Settings > Products.