What format is supported by Superpowered that accepts WAV? Does the audio file size of that format is really small?

WAV is not small, as it has uncompressed, lossless audio. That is the only format Superpowered supports for writing into a file. Superpowered has the best and highest performance decoders, but no encoders. There are no "abstractions" and "graphs" with Superpowered, every raw data is accessible. This way you can integrate any other encoder into your audio pipeline.