You should be OK driving the Prius. I'm 6'4" too, and I have more than enough leg and head room. My only complaint about driving comfort: the seat bottom could provide a little more thigh support (it's not vertically adjustable). But I've taken trips as long as 300 miles, and with regular breaks I've done fine. Other tall drivers have reported some discomfort, especially on long trips. I can't say whether it's better or worse than the Civic; I never test-drove the latter.

As Bob mentions, the Camry is an option and will be a more comfortable and roomier ride, but for more money and lower fuel economy.

Rear seat room in the Prius is ample. One of the reasons I chose the Prius over the Civic is for that. My teenaged daughters inherited my height (one is 6' and the other 5' 10"), and they fit in the back seat with no problem. I think your children will be fine, in their car seats now and when they outgrow them later

As bulldog suggests, the cargo space is good, but I don't know how large your stroller is. The only advice I can give is to try it on the dealer's lot. The hatchback is great and it's surprising how much cargo you can carry with the rear seats folded down, but that of course is not an option for you.

Also as bulldog suggests, with a little study, practice, and application of "hypermiling" techniques, you should substantially improve your fuel economy as time goes on. I suggest you search the topic here and on the other Prius or Civic Hybrid forums.

Jim

Lifetime fuel mileage:

After learning how to hypermile: