It's just very tricky. I could tell you to eat this, and this, or eat X amount of calories per day, but to me, without knowing exactly what you've been eating, and what your body currently requires every day to maintain the weight you're at now, it truely can not be done.
If what you have been eating is clean, and consistent, and you think you can lean out with what goes in your mouth now, but not gain any muscle, then add about 200-400 calories. What those calories need to be? Not sure. Just depends on what you're getting now, as far as percentages of Carbs, Protein, and Fats.
For instance, I'm on a meal plan right now. It was ~3900 calories per day, with 55/20/25 % of Carbs/Protein/Fats. I was starting to loose too much size, too fast, for being X weeks out from my first show. So, we bumped it up to ~4300 calories, adding a meal, to a total of 9 per day (some small, some normal sized), and kept the 55/20/25 %s. So it gets to be a science when figuring out what YOUR body needs.
For now, I would just add in maybe one or two extra Protein supplement shakes a day, and see if you notice a difference in 2 weeks, or something. You could get your Body Composition tested, to see exactly where you're at now, and get the same person to test you in 2 weeks to see if any changes. If not, then you need to adjust again, somehow. If so, then keep up with exactly what you're doing.
CONSISTENCY IS THE KEY!!