Currently sprite Grid Slicing has only two fields (X and Y) for defining the size of the resulting sprites. That is not always the best option you can have. It's usually much easier to specify how many rows and how many columns the resulting grid from the spritesheet should have. You can have both X, Y and Columns, Rows options and when you change one you automatically calculate the other. Or just keep Rows and Columns as it's easier to work with and understand.