Working with large slabs of clay can be a challenge.  My students would generally opt for using the slab roller to create their slabs.  However, I have always offered them an alternative, by demonstrating the fine art of hand rolling the slabs.  Though the slab roller is relatively fast, I find I have fewer problems with cracking if I roll my slabs by hand.

First, I wedge my clay regardless of the pristine appearance of the clay in the bag.  Wedge it!  The wedged clay is slammed onto my wedging table at an angle to compress and flatten.  Those flattened pieces are then roughly joined to form a larger slab on a fresh canvas.  Roll out from the center as evenly as possible.  Cover your slab with another canvas, allowing for the edges to be caught together.  Holding firmly to the canvas, lifting up and pulling toward yourself, flip the slab – canvas and all – slamming it onto the table.  It takes practice.  Continue rolling and flipping until you achieve the appropriate size and thickness.