See the only thing that can be extrended, is the back, the legs and arms are muscle based and are elastic so its not usually permanent but what i can tell you is play basketball or jumping jacks or jumprope, anything that can push gravity on your back without causing pain to extend ‘em.
For penis size , once again i say! The penis is not a Muscle like your chest, like your arms like your legs, the penis is vulnerable to damage and sensitivity and not as strong, thats why girls kick ya there’ so doing it softly, like jelqing - “Google.com type in jelqing” you’ll find many matches, even tool sto help.
Good luck sir!