Agree with above. I have done this route many times most recently 2 weeks ago. For that number of people I would go for a mid to full size SUV primarily for the luggage.
The road up to Whistler is generally very good and stunningly beautiful.
All the underground lots have plenty of room for cars of this size.
The most annoying thing is having to pay for a rental and then having it sit unused for most of the time!