I read the users' guide of Mx. The sample size and power can be calculated by Mx. But I haven't learned how to use MxGUI. Could OpenMx be used to calculate sample size and power?
Yes, but it is a bit more "manual" than it is with Classic Mx. See the presentation and scripts in this workshop directory to get you started: http://ibg.colorado.edu/cdrom2010/bneale/Power/