On the spectrum of mid-size cars, the Buick Regal sits squarely in the middle. Its back seat and trunk don't net as much space, though, as some other popular mid-sizers. It's still comfortable for four adults, provided the back-seat passengers aren't larger people.
At the midpoint in wheelbase and overall length of compact and full-size--think Ford Focus and Fusion--the Regal doesn't come up shy in front-seat space. Base models have more mildly shaped seats than the GS' buckets--they have much better bolstering--but they're all trimmed in leather. We'd ask for more support on the base Regal's bottom cushions, but head and leg room aren't constrained--unless you slide the seat forward and opt into the sunroof.
The Regal's back seat is rated for three people, but two adults could have trouble fitting in. Leg room is similar on the spec sheet to that in the Honda Accord, but something about the cushioning in the Regal makes the back seat seem less spacious--while it's still about three inches more generous than the same space in the Acura TSX. For sure, the roofline meets more heads than in taller sedans, but it's comparable to truly large four-doors like the VW Passat in that respect.
Cargo capacity isn't the Regal's strongest suit, either. With just 14.3 cubic feet of trunk room, it's about 3 or 4 cubic feet smaller than other sedans, and a piece of luggage less than the enormous 20-cubic-foot trunk in the Ford Taurus.