If you already have the chickens, then all you can do at this point is up the protein intake.
 
The size of egg your chicken is going to lay is determined by genetics and how it was fed and what kind of lighting it received during the first approximately 28 weeks of life.
 
If you don't have the chickens yet, buy chickens that are supposed to lay jumbo eggs, feed them properly and well, provide proper lighting and you're all set!
 
Example of lighting issues.  After 8 weeks of age, don't leave the lights on all night.  It messes up their pineal gland.  Things like that. edited to add:  If given too much light they will reach sexual maturity well before they have gained the necessary size.  When that happens, the eggs are smaller when they start their laying and they eggs won't get as big as they would have had they been raised in natural lighting.  The pineal gland can register light even with the eyes closed.