I've taught classes 20+ people classes and 3-people classes. Honestly, my optimal number is around 10. The reason is because the students actually work in their own groups and form a 'team'. When set up against another 'team', the competition can help drive students. Of course, students can crack under pressure, but that is less likely to happen with a larger class (effect of an outlier in a larger sample set etc).
What I also enjoy seeing is students 'arguing' with each other over what is the right/wrong answer because it allows them to 'discover' their own flaws, which I believe is a much better way to learn than by being told what the flaw is. Much like how I could tell you not to stick your finger in the toaster because it's dangerous, versus you actually getting electrocuted because you decided to get curious. Of course, you would never make that mistake again because YOU were the one to discover the mistake.
In terms of attire: I always wear jeans and a t-shirt when teaching at home, or jeans and a collared shirt when teaching at work. Too formal with suits, black leather shoes, black pants etc could create a strong image of authority, which will prevent students from asking questions because the tutor is 'scary'. I believe that authority in terms of teaching should be EARNED through your demonstration of your abilities, not through your attire. Though that is not to say that genuises can dress shabbily. Some decency is of course required.
If I had to name (in my opinion), the three most important things required in a tutor, they would be:
1. Friendly (more approachable, students ask more questions).
2. Knowledgeable, and perhaps even beyond the syllabus (students have confidence in their tutor)
3. Dedication (students feel cared for)
When I teach, I like to use a lot of analogies. The reason is because although the students may not understand a concept of Mathematics (say a limit), they definitely do understand the concept of an armwrestle (which works much like limits). So why not utilise their 'prior' knowledge to assist in the teaching of a concept? They understand a lot more easily and your job becomes easier: win/win.
Another good thing (in moderation) is humour. Reason is because it reduces tension and I utilise it to help teach topics by making incorrect things seem so ludicrous, it is humourous.
For example, if I said to you "If you type 'Google' into Google, you will break the internet" (I love IT Crowd), you will probably start laughing at my face because it is just so silly because it is obviously impossible. Similarly, for Mathematics, I've managed to get students to be able to laugh off silly things like finding 'negative area' for integration, for finding 'negative time' for time problems.
There are a lot of things I have to say about education, but cannot just condense it into one post. If you have any particular questions, I will answer them, but I do sometimes find it difficult to make a 'general post' like this one.