Best workout for a wider chest like phelps?

Cross training/swim: Cross training to include swimming, weightlifting and other aerobic activities is a great way to improve your fitness and secondarily a chest like phelps.

Start at age 12: To be honest, the body adapts during early adolescence. Swimmers see the most dramatic changes between the ages of 11 and 16. The expanded lung capacity and large chest is the result of this adaptation. After age 16, we are really unable to to change the width of our chest. The best we can do is develop our musculature with a balanced core exercise program along with weight training regimen.