It looks like the cell width & height can be anything you want. The software just divides the width by 4, and the height by 11.
So, take the largest image in your spriteset, add a few pixels for a margin, and use that as a guide.
So, let's say the largest animation image is 51x48. use 53x50 for each cell.
Your total spritesheet size would be 204x528.
Then in your image editor, assuming it supports a grid, set your grid to 53x50, and make sure
each image stays inside it's own cell.
I'm not sure about individual alignment within a cell, but I would think you would the actors center of mass in the center of each cell. Probably could use the head as a reasonable guide.
The screen location (reference point) of each cell as it's drawn will be x = cell_width / 2, y = cell_height. So a point at the bottom / center of the cell.
Does that help?