Is there a way to shrink canvas to image size

[userwords]

In a multiple layer image with alpha in the space not used, is there any option or plugin that could shrink canvas size to fit the space used by the images pasted in layers and in their positions.

[Red ochre]

Try this:
1. Magic wand  to select the transparent areas. Use the selection modes to select all the outer transparency. (You can select on different layers but this won't be relevant for what you are trying to do.)
2. Edit/Invert selection or (Ctrl + I). (the solid areas should now be selected with a pale blue tint).
3. Crop to selection.   - this should give you the smallest rectangle you can crop to. You will still need to save the flattened image as '.png' to keep the smaller transperant edge areas.

 

Hope that helps.

(Btw plugins cannot change the canvas size)