MLPowSim is a piece of software for estimating sample size by simulation for data to be modelled using GLMMs, including for instance logistic regression with crossed random effects. It spews out R (and MLwiN) code for doing the simulations. Screenshot below:

Haven’t had time to play much yet, but you answer a load of questions, e.g., what sort of model, how many predictors, what random effects, estimate for slope, etc, etc. Looks useful! I suspect it might be easiest to generate a few example R scripts using the package and then hack them by hand. I stopped when it asked me, “There are more than one random effects associated by the first factor in your model and so you need to enter variance and covaraince matrix.”