Barring some kind of genetic abnormality, you will find almost no examples of a person accidentally gaining too much muscle. In fact, the vast majority of people who want to gain a lot of muscle still find it very difficult to do, because it requires not only an extended dedication to working out, but also a diet consisting of the right types of food -- often more of those foods than they would normally prefer to eat.
To put it simply, avoiding muscle growth is not something that requires any special effort. If your current diet allows you to maintain a "slim" build, simply maintain that diet while following whatever strength training routine you choose, optionally adding a healthy snack to offset the energy you're burning during exercise. The biggest threat to your appearance is probably increased appetite and how you choose to respond to it. If you're not eating specifically to gain, you will most likely gain very little, and what you do gain will come gradually.
There's some fine advice here already on exercises, but make sure you don't ignore the role food plays when it comes to any fitness goal.