Is there a way to automatically resize the canvas to the combined size of all layers?

Hello there,
for what I know I don't think there's a specific command for what you asked.
I think I would have used this feature to reduce the canvas to the overall size of the painted areas, instead of going through manual selecting and cropping.

There's a feature in Inkscape which performs a similar task very well and neatly (you can manage the margins, even). But I guess it requires a different approach with vectors.
In case of pixels, maybe it's by evaluating the color changes starting from margins, in order to get the clues on where to crop the image.

I am not sure it is exactly what you asking for, but it is a command
image/trim to current layer
It is kind of mvowada example with reduce the canvas

1. you can put all layers in a group,
2. select that group
3. apply the command, and will crop everything is empty part to painted areas in the group.

If you want to resize the canvas to fit all layers which are bigger and out of the canvas, in my experience no command for this.
Maybe that could help for you some how.