I have one of those 'starter kit' cages that they sell (one of the larger ones)... I use it to carry the boys. I don't really like to put more than two of them in it, though, even for a very short trip. All cramped.
The thing that I've noticed about carrying cages is that the lower the ceiling, the happier they are to just lie down and sleep. Then they don't feel compelled to climb & hang upside down, trying to get out.
Also, I think any carrying cage needs to have a large door so that your rats can't cower in the corner, eluding being picked up (and wasting the vet's time).
There are some more rectangular carrying cages available at many pet stores for around $10-12, where the entire top opens. Those, imho, are a lot nicer than the one I got.
And having a nice plastic pan for the bottom is really important - you don't want poop & pee on your carseat.
That's definitely plenty big - a travel cage doesn't need to be anywhere near the size of a regular cage, provided that you're only keeping the rats in there for under 2-3 hours, especially during the day when they're less active.