We have 10K in an account at ING Direct, or whatever they're called now. We are a Foreign Service family so our expenses are very small. If we work private sector again, or a more traditional government job, I will up it to whatever 4 months of expenses are at that time.
I don't expect a job loss because our job is fairly secure, government shutdown not withstanding.

It's meant to cover a medical emergency, a need to buy plane tickets home, or something like that.
We also own 5 rental properties, and I keep a 30K fund for those. It is designed to cover a new roof, a flooded basement, something like that.
HTH!