Go into Settings -> Shipping, then scroll down to “Package” and press “Add New Package”.
In the new window set your Package Name, then set Default to Yes and finally fill in the package weight and dimensions.
You can also press “Add Default Package” to automatically create package presets for standard USPS parcels.
You can also enable automatic weight calculation in the same place. This will override your package weight settings and the system will calculate the weight for each order once you're ready to ship it.