While using ReadyToShip to process order via MyPost Business you have the option of selecting 'Own Packaging (or custom packaging) or using pre purchased Australia Post Packaging boxes and satchels (these come in sizes from S to XL - available here : https://auspost.com.au/shop/sending/boxes-packaging?fm=recommendations:shop:3).
When you add MyPost Business as a carrier connection in ReadytoShip we automatically add the standard Auspost Packaging sizes in the Packages section in ReadytoShip (Settings > Packages). These cannot be edited.
If you use MyPosts defined packaging you can select them for your order using the package menu as seen below.
Selecting one of the defined packages will auto fill those dimensions in the order and either use item weight (if imported from your store/marketplace) or default weight.
(see this help section on Packages and Weights/Dimensions)
Note: If you edit these dimensions (Not weight) in any way they will become Own Packaging and Auspost will calculate rates differently.
For the latest MyPost Business rates and fees see this link.