I am leaning towards purchasing another crate.. does anyone have any input on the smaller size I was considering? 24Lx18Wx19H I think. I know shelties size varies so much so it's probably hard to say. It says on the website that it should fit a dog up to 25 pounds but 24 feet long seems so short!
Right now I think I'm going to order another crate to stay in the car, for the normal shopping, training classes, errands, and then also purchase a harness, when he's grown, for longer trips (10+ hours) because I think it would be quite cramped in my car with our luggage, camping gear, crate, people!, etc. Maybe what I really need is a new SUV to replace my little car

Thank you for your input.
Ha... this is pretty much exactly what we did!
We started out doing just what you're doing, lugging the dog's crate into and out of the little car (in our case, a '91 Sentra, with only 2 doors). We did this on a daily basis, since we walk the dog at a park nearby. So, we decided to get a second crate to just stay in the car. We didn't get a wire crate this time, though. We got a collapsible travel crate. Well, if I had to do it again, I'd probably get a wire crate, simply because I'm pretty certain that the collapsible travel crate would not be any better in a crash than if the dog was just loose in the car.
The travel crate was a bit smaller than his wire crate (his wire crate was the same dimensions you're currently using, the travel crate was about the same as the smaller one you're considering), and it was just fine. All he did was sleep in his crate while in the car, and it was still big enough for him to do that. So, you might be just fine with that smaller crate.
You're right, it's very hard to know how big they're going to end up. Lerwick started out a bit small, and ended up oversize. He actually got too tall for that 30" wire crate, so we got him a 36" for here at home. That said, we still use the travel crate when we travel with the dog, because he still has plenty of room in it to lie down and stretch out, and he is never in it for long.
We also tried two different harness restraint systems. They're fine for those short 10 minute drives to the park, but not good for long trips. The first one (
http://www.kurgostore.com/big-gear-sale/tru-fit-smart-harness-seatbelt-loop/) was so restrictive, Lerwick couldn't even lie on the seat. Just had to sit bolt upright. We never even tried that one for a long trip. The second (
http://www.kurgostore.com/dog-travel/kurgo-auto-zip-line-leash-zip-line-combo/) wasn't restrictive enough. In our little Sentra, we had to set it up as you see pictured in the second photo, and in order to have the leash long enough that he could actually lie on the seat, it was also long enough that he could jump onto my lap in the front seat! He was just all over the car, and usually ended up cowering behind the driver's seat on the floor.
So, if you're thinking of getting a second crate for short trips, and a harness restraint system for longer trips, I'd actually recommend you skip the harness and just stick with the crate for all trips.
Or, you know, do what we eventually did... bypass all the make-do solutions and buy the dog an SUV (or in our case, a Subaru Impreza wagon), install a pet barrier behind the backseat, and turn the cargo area into a big crate for the dog!
We call our cargo area/crate "Lerwick Land."
It is QUITE a bit more expensive than either a second crate or a harness he might outgrow... but it's effective!