The drain current in MOSFET is directly proportional to width and inversely proportional to its channel length. So to have large drain current MOSFET width is kept larger than channel length. In a standard cell library there are different cells available for same function(e.g. NAND gate). They differs in their driving capability. Larger the width of transistor, higher the drain current so higher driving capability. But larger width causes larger gate capacitance which can cause larger slew at input. So depending on the requirement transistor width can be 2X,3X,4X of channel length.