As far as tips for Tiffany Town Car...tips are up to you. Typical tips are $1 per bag. If you have a big family you have more bags hence a bigger tip.
There are only two in our party (Wife and I) when we go to WDW. We have only have 2 bags total but we tip $5.00 each way when we use Tiffany Town Car.
Remember: Tipping is not mandatory...it's a gratuity for services rendered. If you feel you have had GOOD service...tip GOOD...if you feel you've had GREAT service..tip GREAT and if your service was miserable...don't tip at ALL. But if your service was bad, be sure to let the company know so they can correct the problem.
In July we had 5 people with 3 bags, 1 huge and usually give the guy a $10.00 tip each way. Tiffany guys are usually very friendly and our 3 DS always ask lots of questions.