If you want to Gain size and remain lean you'll for starters you need to have the right diet. I would make sure you eat small-medium sized meals every 2 - 2.5 hours. Each meal should contain a lean, high quality protein source (25-50 grams, depending on your body weight), a clean, low glycemic, complex carbohydrate source (approx. 50 grams), and a green fibrous vegetable. This is the time to write which foods you eat in your journal to see how your body reacts to them. Remember you need to learn what works for you to really start tapping in to your potential.
TRAINING
As far as training goes, overload that muscle with progressively heavier and heavier weights. Give that muscle a new stimulus to adapt to by growing. Try not to be in the gym for any longer than 1 hour and 15 minutes. I would even strive to get an intense weight training session in as little as 45 minutes. If you find you are going over that mark try doing less sets but going full intensity for each one... 6-8 reps have worked well for me in the past. Again your body may be different so try 3-6, 4-8, 8-12, until you find what is working best for your body
Use The Mirror!
Yes, this is another must. DO not get on the scale to help determine if you are gaining muscle. Use the mirror. If you see you are getting a little smooth, you must adjust your cardio and diet accordingly until your physique reaches the leanness you desire. Vice versa if you feel weak and lethargic, boost you calories slightly, by about 100 a week, until you tighten and fill out. Do not remove the cardio; just eat more clean food.