To Straighten an image:
- select the "Measure tool" via shift + M
- mark a vertical or horizontal line on the image by drawing it using the mouse
- set Interpolation to "NOHALO" & Clipping to "ADJUST"
- click on the "Straighten" button from the left menu
- then choose the menu option: "Image → Fit Canvas to Layers" if necessary
Resizing the Canvas:
If you want to adjust the canvas size to fit your newly rotated image, you can increase the canvas size to include all parts of your image. To increase the canvas size, you can click on the chain icon next to the width and height values to link them, and then enter a larger value for either one. This will resize the canvas proportionally and center your image.
Alternatively, you can increase the canvas size to fit the new image by selecting the menu option: Image → Fit Canvas to Layers
Source = forum
