Upload a photo of your wall, set the scale, and see a real piece at its true size. Pick a piece, try a few sizes, drag it where it would hang, and save the preview to share with anyone who has a say.
Take it straight on, with the wall filling the frame. Your photo stays on your device and is never uploaded.
Drag the two blue dots to the ends of something you know the length of, then enter that length.
Size is the longest side.
Drag the piece where it should hang. With it selected, the arrow keys nudge it 1 in (Shift: 6 in).
Preview only. Colors and proportions are close, not exact, and perspective is not corrected.
Stand centered, a few steps back, with the camera at about chest height and the phone held level. Angled photos make the piece look skewed, because this tool does not correct perspective.
Skip the ultra-wide lens and portrait mode. Wide lenses stretch the edges of the room, and that throws off the sizes.
For the scale, pick something that sits in the same plane as the wall: a door frame, a window, the wall's full height. A sofa is fine if it is pushed flat against the wall.
Put the dots exactly on the ends of the thing you measured, then enter its real length. Zoom the photo on your phone first if you want a more exact placement.
Aim for the center of the piece at about 57 to 60 inches from the floor. Over a sofa, leave 6 to 10 inches above the back, and keep the art about two thirds the width of the furniture.
Compare a size down and a size up. The 24 inch and 60 inch jumps look very different on a real wall, and one of them usually feels right at once.