It sounds like you should improve on your overall size and speed, in addition to position-specific skills, of course.

I recommend playing another sport such as track and field. Doing so will help improve your speed tremendously. But until that season, you'll definitely want to dedicate yourself to a strength training routine. Here are some guides that will help you with that:

http://amfootball.isport.com/amfootball-guides/football-training-in-the-offseason

http://amfootball.isport.com/amfootball-guides/football-training-with-olympic-lifts

http://amfootball.isport.com/amfootball-guides/football-agility-endurance-exercises

Also, check out the Videos and Guides sections for some specific drills and workouts you can do to improve at the wide receiver/corner back positions.

Let me know if I can help with anything else!