I'll come at this from a different angle, we "upsized" to a 1456 sq/ft house from the 768 sq/ft mobile and apartments we used to live in. This size seems perfect for a couple. We have three bedrooms, one is our master, one is my home office, the other is my wife's craft room/guest room. Plenty of room for privacy and moving around, but small enough to keep clean and heat efficiently. Most important, we have no wasted space. Every room in our home is used almost daily.
As others have already said, all one level, 36" wide doorways, curbless shower (ours has no doors or curtains either).
High or vaulted ceilings really help make a small space feel much larger.
Walk-in closet in the master is much better than bi-fold or sliding doors. Our built in dresser and shelving in the closet saves a lot of space that would otherwise be taken up with furniture in our bedroom.
Minimize hallways in your floor plan. They just waste space. We only have a small 6x6 "hall" room in the center of our house. Also, consider furniture when designing a floor plan. I designed ours with a straight shot from the front door to any room in the house, no tight hallway bends to try and get a couch or mattress through.
Aim for as "square" of a house as possible. Our 40x40 house has 160 linear feet of wall space, while a nearly equivalent 20x72 house would have 184 linear feet. That means more materials to build, and more area for heat loss. A long narrow house usually means a long space wasting hallway too.
A kitchen doesn't need to be huge, it just needs a good flow and work triangle. I always have to laugh at those huge kitchens on HGTV shows where the fridge is 15' away from everything else. That will be horrible in daily use. I built drawers in all of our lower cabinets whenever possible, much easier access. Our kitchen is only about 10x12, but we have more counter space and easier access than a lot of much larger kitchens. Make sure all the doors and drawers can open without interfering with each other.
Definitely two bathrooms. Or at least 1 and 1/2. Even with just the two of us, two toilets are a must.
Go all electric for heating, hot water, AC, cooking, etc. Gas will be phased out over time, and electric gives you many options (solar, wind, hydro, battery, generator, etc.). Not to mention gas reduces indoor air quality, especially in new tighter homes.
Have redundant heating systems in case one fails. We have in-wall electric wall heaters, a mini-split heat pump, and a wood stove.
If possible, add a battery backup system for power outages, especially if you live in a rural area.
Install electrical boxes with conduit runs to the crawlspace and/or attic. It's impossible to predict what wiring technology will bring in the future, and conduit makes it easy to run new cable, ethernet, fiber, or whatever as technology changes. I've changed our cable runs a few times over the last 19 years, and have never regretted installing them. More would have been better.
Location is a personal choice. Some folks want to be in-town close to the all the action. I prefer to live in as rural of an area as possible. We're about seven miles from the closest grocery store, and about 30 minutes from the nearest hospital. Unfortunately, our area has grown up too much over the years. We have too much invested in our current home to move, but if I was doing it over I would move even farther out if possible. Especially if we were retired and didn't need to make the daily commute.
