Presence doesn't require size
A piece doesn't have to take over the wall to get your attention. It needs to make sense from where you'll actually see it every day.
A 36 × 24 inch horizontal work can work at the end of a hallway or above a console. A 24 × 36 inch vertical work can hold its own in a dining area, entryway, or office.
Bigger isn't automatically better. Go too large and the work can crowd everything around it. Too small and it can feel disconnected from the rest of the room. Start with the wall you actually have.
Measure the wall first
Before you acquire anything, measure the open wall. If the piece is going above furniture, measure that too. Pay attention to doors, windows, light fixtures, and anything else that affects the usable space.
Then get some painter's tape and mark the dimensions of the artwork directly on the wall.
This is one of the easiest ways to check scale because you're looking at the actual footprint instead of trying to judge it from a photograph on a screen. Leave the tape up for a little while. Walk into the room a few times and see how it feels from the places where you'll normally see the work.
Think about where you'll see it from
Distance changes what you notice.
Up close, you can spend time with the smaller details and the surface of the work. From farther away, you're more likely to see the overall composition, the face, and the larger areas of color first.
That's why I think about the room along with the dimensions. At the end of a hallway, you may first see a piece from several feet away while walking toward it. In a dining room, you might spend an hour sitting across from the same wall. A bedroom gives you another kind of relationship because you'll see the work repeatedly from many of the same positions.
Those aren't interchangeable situations. The size needs to work from the distance where the piece will actually be seen.
Give the work some space
You don't have to clear everything off the wall, but give the piece enough room to be seen.
Look at what's going beside it and across from it. Another large artwork, a mirror, shelving, or even a strong light source can compete for attention.
Sometimes moving one object is enough. You don't necessarily need a different piece or a bigger wall.
Horizontal and vertical work differently
Orientation changes how a piece sits in the room before you even get close enough to look at the image.
A horizontal work can make sense above a console, bed, or low cabinet where the surrounding architecture already runs from side to side. A vertical work can fit naturally on a narrower wall, between architectural elements, or in a space where you want to use more of the wall's height.
Many of my works are 24 × 36 inches. I like that scale because it gives the face enough room without requiring an enormous wall. Some pieces work better horizontally, so I use the same proportions at 36 × 24.
I don't think orientation should be chosen by habit. Look at the piece and look at the wall.
Hanging more than one piece
If you're putting several works together, decide which one you want people to notice first.
The pieces don't necessarily need to be the same size. In fact, different sizes and orientations can help establish some order on the wall. A larger horizontal work might sit with smaller vertical pieces, for example.
Before making holes in the wall, lay everything out on the floor or use painter's tape to mark the arrangement. Check the spacing as carefully as you check the dimensions of the individual pieces.
Scale should be accurate
If I'm showing you what a piece could look like in a room, the proportions need to be accurate. Otherwise the mockup isn't useful.
That's especially important when I'm working with interior designers and hospitality teams. If the wall dimensions are available, the artwork can be shown at the correct scale before anyone makes a decision.
A mockup should help you judge the placement. It shouldn't make the artwork look larger just to make the image more impressive.
What I'd want to know about your room
If you asked me which size I'd choose, I'd want to know about the room first.
Where is the wall? What's underneath the work? How far away will you normally be when you see it? Is it somewhere you pass throughout the day or a room where you sit for long periods?
Then I'd look at the dimensions.
Measure the wall. Mark the size with tape. Walk away from it and come back. That's a much better way to choose scale than trying to guess from a product photograph.