It’s important the shrimp you’re cooking is all the same size for even cooking.  The key is not to overcook your shrimp.  Overcooked shrimp is stringy and chewy.  You can produce juicy tender shrimp in different ways either with high heat for a short amount of time or lower heat for a longer period of time.  For the quickest results though, I would recommend searing the shrimp in a single layer in high heat for three to four minutes turning once.