I have a cage in the bedroom - my two "littlies" sleep in that one overnight - one in the kitchen - which Jack and Rupert tend to use during the day, if I'm in the kitchen, the door is always left open on that one - and one in the car, I agree that this is the safest way of transporting dogs in the car, as long as you also use those bungee straps to secure the cage/crate to the car {most cars have places to secure them}
As for size, well ONE Cavalier would fit in the small size cage, 25"L x 19"W x 21.75"H
These are about the best made ones I have found - as Karlin pointed out, the cheaper ones can have sharp edges
{UK}, this is E-bay
http://cgi.ebay.co.uk/25-Dual-coate...W0QQitemZ230021950902QQihZ013QQcategoryZ20745 QQrdZ1QQcmdZViewItem
but they also have a website
http://www.doghealth.co.uk/
I've had one large Cavalier and two small ones in this size very comfortably
31"L x 22"W x 24.5"H
http://cgi.ebay.co.uk/30-Dual-coate...W0QQitemZ230021951013QQihZ013QQcategoryZ20745 QQrdZ1QQcmdZViewItem
but two of mine are large and one is small, so we've gone up to the next size for car travel.
The free bedding with these isn't very thick, I actually use it underneath the plastic tray which stops it rattling!
Rupert is the only tiny puppy I've had, I house trained him with the help of a cage and it was brilliant, so easy. He only went in it when I couldn't watch him or overnight. He still loves his cage now...sees it as a den, especially with a blanket over it.
I would always crate train any future dogs, it makes it so much easier when you go away, also if they ever have to be crated following illness or surgery.
I also have those fabric type crates, they are ok, but only if a dog is used to being crated, and I'm not keen on them in hot weather, the dogs seem to get too hot.
If and when we have another puppy, I have a crate with a metal wire door {the cheaper ones are plastic which can be chewed
} which I would use for transport and overnight....