To scale a mesh to fit an image in Spine, you can adjust the vertices of the mesh manually. You can box select the vertices and move them to fit the image. However, Spine does not currently support rotating or scaling vertices directly in edit mode. If you need to adjust the size of the image itself, you can resize the image in an external image editing program, and Spine will adapt the mesh to the new image size. If you are using a larger image, Spine will prompt you to adjust the texture coordinates to keep the image contents the same size.
For more detailed steps, you can refer to the Meshes - Spine User Guide: Image resize.
