Measure the frame
Take the backing board out of the frame and measure it. That is the size the paper has to be. Write it down in millimeters.
Frames sold in inches convert at 25.4 mm per inch. An 8 × 10 inch frame, for example, takes a sheet of about 203 × 254 mm.
Open the canvas size
Open your design in the free editor and click the size shown under the Edit button, for example 200 × 266.7 mm. The Canvas size window opens with the current size filled in.
Type the new width and height, then choose under Artwork how the design should follow.
To do it in one step with a file on your computer, drop it on Resize a layered SVG to fit your shadow box frame. The editor opens with the Canvas size window already up.
Scale with the canvas
Choose Scale with the canvas when the frame has the same proportions as the design, only bigger or smaller. Width and height are linked: change one and the other follows.
Every layer, cut and shape scales by the same amount, and the note under the grid tells you the percentage. Bridges keep their width in millimeters, so they still hold the paper after the design gets smaller.
Keep its size
Choose Keep its size when the artwork is already the size you want and only the paper around it should change. Width and height can be changed separately.
Use Pin the artwork to to decide which side moves. Pinned in the middle, paper is added or trimmed evenly on both sides. Pinned to the left, only the right side changes; pinned to the top, only the bottom. The arrows in the grid show which way the paper goes, and the note spells it out, for example “Adds 60 mm of paper on the right.”
The added paper is solid on every layer. If a layer was cut open right up to the old edge, stretch that cut over the new strip. When you make the canvas smaller, anything past the new edge stays editable but is left out of the download, so move it back in if you still need it.
Fit a frame with different proportions
If the frame is a different shape from the design, do it in two steps. First use Scale with the canvas until the design fits the frame on its tighter side. Then use Keep its size, pinned to the middle, to add paper on the other side.
The artwork stays centered and nothing gets stretched.
Check and download
Choose Apply. The change is one step, so a single Undo takes it back. Open Preview & download to check the stack before you cut.
The download is at the new size in millimeters. Keep that size when you import it into your cutting software.