We have two kids, and my husband is 6'2"+ so our Jetta wagon was too small with two carseats in the back.
We test drove a Mazda 5 (I loved the gas mileage and maneuverability) but it was just too small. And I had some concerns with other issues too.
If our kids were older it would've been okay maybe, but if we had three kids no way -- using the last row of seats removes all storage, and the trunk portion isn't lowered enough, so anything in the trunk (seats up or down) becomes a potential hazard in a crash. The notion of using the third row for actual seating beyond a 10 or 15 minute trip seems unrealistic to me. And as others have pointed out, using it for a child's carseat seems unsafe to me. :/
Also, I need to be able to nurse the baby in the car. I hate sitting in the front seat to do this (what if someone rear ends us in a parking lot? And the airbags went off?) and wanted the third row so I could nurse her easily. The problem with the Mazda 5 from what I could see is once the carseats were installed I wouldn't have been able to get to the backseat easily. Certainly not while carrying a baby.
I had told my husband ahead of time the one thing would could not compromise on was space. That was the reason we were getting a new car. We had a set pricepoint and were fine with buying used in order to get the space we needed.
After we left the Mazda dealership we were on our way to look at a Kia Sedona (comparable price -- but much bigger) when we found a barely used VW Routan, which we ended up getting a great deal on. (Got it for the same price as the Sedona.)
While the Kia has very good safety ratings and warranties, know that the resell value on it is lower than Any Other Car. Not just minivans, but any other car out there. So if you look at Kias, keep in mind you'll want to drive the car for a very long time in order to get your money's worth.
I have a friend who bought a used Mazda MPV. She loves it. We couldn't find one in our area or we'd have tried it. I love love love our Routan (it's essentially a Town & Country with VW styling). I've always driven really small cars (the Jetta was the biggest one I've ever owned until now) and have had no problems getting used to driving a minivan. If that's your main concern, I wouldn't worry about it at all. I'm petite (5'2") and have had no problems with driving something bigger. It's definitely been worth it because I can nurse the baby in the car and have some privacy and not feel squished.
And we can go to the store and my husband's legs won't fall asleep.
Yes, we sacrificed gas mileage but I don't drive it daily and my husband now drives our super economical Jetta (it's a diesel). So it all evens out in the end....
Good luck, mama! Hope you end up with a car that suits your needs perfectly.