The canvas size for each frame is 80x120(which means the whole sheet is on a 80x120px grid) as written in the asset pack description. It was kept larger, with padding on sides, to be kept consistent across all frames, so that you easily can separate it. The reason for making the canvas 80x120 is because the animations have varying sizes, as you can see the idle frames could fit in some much smaller canvas, because there is not much vertical or horizontal movement. But since the attack animation ends up taking up a very wide horizontal space as well as some other animations, I picked a canvas size that is large enough to fit every frame, as well as keep the character still in center so that it's easy to implement.
So essentially you just need to divide it up into 80x120px grid, and keep the origin centered then, the character will be perfectly centered and everything will fit.