Can i change my muscle size and definition?

Yes: Through diet and exercise.

Exercise/Nutrition: Weight training generally helps to shape and tone muscles while cardiovascular exercise burns the fat around the muscles so you can see them better. Lighter weights with higher reps are better for tone, and heavier weights with fewer reps would be better for size and strength. So you need higher rep exercises, regular cardio and diet of lean protein and complex carbs.

Yes.: The most effective way to build muscle is through progressive resistance training with weights, machines, and other various methods. Muscle definition is more a function of body fat, you can see the muscle better if there is less fat around it. You can decrease your body fat with exercise and diet. Consider consulting a trainer and nutritionist to help map out and achieve your goals.