Individual Sprite Size

Hello everyone,

I want to use Animate CC to create animations in my 2D game so to create sprite sheets. However, I couldnt find a way to set sizes of individual sprites in a sprite sheet (not the size of whole sprite sheet).

Since I want to slice my sprite sheet as 256x256 in Unity, I need to set sizes of each individual sprites as 256x256 in my sprite sheet.  Everytime I generate a sprite sheet, Animate CC sets random pixel sizes (let's say 210x250) for each individual sprites and this is what I am trying to set, I am trying to make these sizes 256x256 so I can easily slice them in Unity.

Does anyone know how to do this in Animate CC.

I've tried to use Guide Lines but did not work. My next plan is to put little dots to four corner of the canvas so I can push Animate CC to generate each individual sprite in the canvas size but this sounds weird..

Thanks in advance.