It really depends on the puppy. I had Willow and Shadow in crash-tested car harnesses from a young age. Squidge, however, ate through two seatbelts as she hated being restrained
The best crates you can get are crash-tested ones like the Trans-K9 ones; they are the safest sort to get. Our
@Heidrun was involved in an accident with her dogs in the car, and she says how she is sure the crate saved their lives. Her story is here:
Crash Tested Customer Stories for Dog Cages & Boxes | Transk9
For dog guards, you could look at Travall, which are crash tested.
A smaller space (within reason!) is safer for the dog than a larger one in the case of collision, but you have to temper that with how much time she will be in the car, and obviously the practicalities for you. Depending on what vehicle I'm using, my dogs travel either in the boot, behind a crash-tested car guard (and in a car with a 5* safety rating, which is at least as important as the device you're using); or in crates in my van - I am yet to get my custom boxes made, but that's the idea when I can; or occasionally tethered in crash-tested car harnesses.