If you're looking for a big car that gets great mileage, look into the New (2012) VW Passat TDI. We have almost 12k on ours already (bought first wk of March!), are averaging 40+ MPG, and I fit in the back-seat for short trips, and am 6'6" and 350-ish lbs. Driver's seat is my fav place to be, and it runs and drives great. Not a lick of trouble, and the first 36k of maintenance is on VW. The hardest part about them is finding one on the lots, they're flying like hot-cakes.
It also should accommodate 2 car seats easily in the back, and is 5 star crash-test rated, which is a huge plus. Ours is the SE + Navigation + Sunroof, but you can get a "base" SE with the TDI and manual, otherwise the DSG transmission (auto-shifted) is excellent as well, which is what we have.
The car is also paying for itself in fuel savings over our '11 Suburban, since that's pretty much sat in the garage since we got the Passat.
BTW, the Nissan looks great as well.
