I like the wire crates with doors at both ends as they are more sturdy than anything else I have found. The sherpa crate pad fits perfectly inside the crates and doesn't let them slide around while in the car.
Small crate (20 lb or less) - 24x18x21
Medium crate (20-35 lbs) - 30x21x24
Medium/Large crate (35-55 lbs) - 36x24x27
Large crate (over 55 lbs) - 42x28x31