With growth models often a smaller sample size can be used. The only way to truly know if your sample size is large enough is to do a Monte Carlo simulation. See the following paper which is available on the website:
Muthén, L.K. & Muthén, B.O. (2002). How to use a Monte Carlo study to decide on sample size and determine power. Structural Equation Modeling, 4, 599-620.