Hey there Jericho,
Great to hear you're looking to go the immersion route, no better way to rapidly pump up that knowledge (and have an awesome time doing it!)
1. Class sizes seem to vary between 10-20 students, based on your level. I recall 6 being the minimum amount required to create a class level, but that small is atypical.
2. 12-15hrs/week is definitely doable with a full load at Naganuma as I worked about that much (remotely on US-based job). You likely won't be able to start work/volunteering immediately unless you have an opportunity already set up for you before heading there. I also wouldn't recommend starting immediately as the pace of school and life took me about a month to really get used to.
Also, you're probably already looking into it, but make sure when evaluating schools to ask about JLPT prep the school offers. While Naganuma lessons aren't directly designed for passing JLPT, I recall there being prep groups they help set up. It's great to walk away with a piece of paper validating our knowledge as many companies require at least JLPT level 2 for any serious forms of employment.
Enjoy man and please ask any other q's, glad to help use my limited experience.
- Jeff