Specifically it's .46 MB per minute assuming that, like most audio podcasts I've seen, it's encoded at 64kbps mono. If it's a stereo podcast, or at a higher bit rate, it will be bigger. As for video, I'm not sure as I've never downloaded a video podcast. The only way to know for sure is to just download it - you can always delete it.
Either way, it sounds like the 8 GB model will be fine for your needs.