The prefetch size (bytes) in the converted file is not accessible with WAAPI.
You can access the value in the SoundBankInfo files (json/xml), generated with the Soundbank files:
We also plan on adding WAAPI a topic you can register to be notified when SoundBanks are generated, containing the prefetch size:
The prefetch lenght is accessible directly as a property on the Sound objects, see here:
PreFetchLength: Prefetch length (ms)
To access the value, use ak.wwise.core.object.get on Sound objects, with "@PreFetchLength" in the return options: