full size stackable pair
I have the Frigidaire 2140 and matching gas dryer in a stacked configuration in my garage. I have plenty of room, but there is a bollard (concrete filled steel pipe) embedded in the concrete slab right in front of the washer space, which would prevent a front loader from opening. The pipe is required by county building code to prevent a runaway car from taking out the machines (or the adjacent water heater or furnace....). When the home was built in 1988, there were no domestic frontloaders, so they probably didn't pay much attention to the placement. Had they located the pipe right at the juncture (midpoint) of the washer and the dryer spaces, they could have afforded protection without interfering with a washer door.
Be that as it may, I bought Frigidaire front loaders that can be stacked. I bought a stacking bracket for all of $30 and the pair is stacked in a freestanding configuration against a wall. So far, doing great. I purchased these machines 18 months ago.
Be aware that most of the larger machines (above 3.5 cu ft) achieve the extra size by being wider from front to back. They may be only 27 inches wide, but some are 30-31 inches deep, and often up to 33-34 inches tall (meaning that the top of a stacked dryer can be six and a half feet off the ground....ok if one is tall, but not good for shorter people).
Built in installations with close tolerances (i.e. a laundry closet with a door that hides the machines) may not work with the larger machines. My sister has such an arrangement: it's a wide closet with room for side by side installation, plus overhead cabinets and a sink, but the closet--conveniently located right off a hallway--is closed off by sliding doors...and an extra deep (e.g. Duet) machine would prevent the doors from closing. She is fine as long as she sticks to machines with the traditional 27 x 27 inch footprint.
The Frigidaire machines are I believe the largest capacity you can have while remaining with the 27 x 27 footprint. The dryer is often criticized for being only 5.8 cu ft (I don't find it a problem, if I dry a king size comforter I just have to turn it once), but the flip side is that the dryer will fit in any standard size space, unlike a Duet which needs 4 additional inches of depth clearance.