For development environments we typically target 2 GB instance size, but it has to also run various other services on it (we're in the process of optimizing these), so a somewhat safe limit for dev environments at present is around 1 GB of memory, and once we're done with optimizations, it'll hopefully be more around 1.4-1.5 GB of memory.
For staging and production environments we've assumed roughly 2 GB per 5kDAU, but can tweak things around based on the specifics of the game in question.