To change the length, change the "remaining" number at the top left.

And "num_columns" at the bottom right(half of the remaining number).

To continue from a previous long image, copy the 512x1024 block off the end of the previous long image and paste it on the left of a new 1024x1024 transparent image.  At the top left, add a "load image" node at the start instead of the "empty image" node connect the image & mask to the same place.

Need to install custom nodes: https://github.com/BadCafeCode/execution-inversion-demo-comfyui