Try to use
Tools | Resize
or
Tools | Canvas size
Resize vs Canvas Size
What’s the difference between the Resizing and Canvas Size functions?
“Resize” command allows you to resize the entire image.
“Canvas Size” command allows you to resize the canvas only (background), without resizing the image.
Available programs:
- Advanced TIFF Editor Plus (free download, home page);
- Advanced TIFF Editor (free download, home page);