Thermal crude cracking is done usually with heat and pressure – in this case it will be based on random chance and fractional distillation will be able to separate the products by boiling point. The boiling point is correlated with carbon chain length due to smaller chains having fewer dispersion forces hence less energy is required to break these intermolecular forces.
Catalytic crude cracking will break alkanes into alkenes through passing the alkanes in a vapor form over a bead of low density catalysts such as zeolite. This results in them molecule splitting hence liberating to alkenes. Repeating this process until the size molecules that are desired can introduce some selectivity as to what the products will be.