Stackable washer and dryers can really be a lifesaver when it comes to space. But watch out! You still want a full-size washer and dryer. I had a stacking w/d in an apartment, and I had to run a load of wash *every day* to keep up with a 2-person washload. The energy bills noticably increased because the small w/d unit wasn’t nearly as efficient as a full-size set. I recommend staying away from the decreased-size washer/dryer units.
Also beware of the inital cost. These units usually cost more for the convenience of stacking them. Sometimes the cost is definitely worth it, sometimes it isn’t. Consider a traditional unit, and then perhaps installing some cabinetry overhead for storage if space is a problem.