Calculating shipping costs for retail. Help please!

I know this is a really dumb question but please bear with me. I have read every article and am still struggling with setting up shipping costs. I own a retail shop and want to sell items on squarespace that vary greatly in weight and size. I'm trying to figure out how to set up shipping and not lose money. If I integrate with a shipper such as ShipBob or Easyship and go with carrier calculated rates then do I need to know the weight and dimensions of the items AFTER they are boxed up? Meaning, would I need to have specific box sizes for each product in order to charge consistantly the same? Sorry, this is one area that I'm having a tough time with.