How do I determine the package size /dimensions for an Order or the individual items?
It wont be hard, you can ask supplier from Aliexpress or Alibaba for the product weight and volume then input them into Shopify, the weight will be sum up once clients choose multiple goods in your store. If you want more easier you can add the shipping cost to the product price then provide free shipping when clients order more than $100 or more.
To ask Aliexpress or Alibaba is assuming the product is sourced or shipped via them ie thet the 3de party has referance to the physical Item.
If the fields where availble on the product page the shop admin could have provided it.
Then this info could be used to supply better shipping rates that are based on density(volum/weight).