I want to offer Free or Flat Rate shipping to different regions or under certain situations in Shopify, is this possible?

The Smart Send Premium app for Shopify is designed to return real time shipping quotes at checkout (if you have the Carrier Calculated Shipping option enabled on your Shopify plan).

If you prefer to simply offer free of flat rate shipping options at checkout that you've set up in Shopify>Shipping&Delivery section in your store admin, you can still do this.  In this case, you don't need to add dimensions to your products in the My Products tab in the Smart Send Premium app (and you also don't need Carrier Calculated Shipping on your Shopify plan).