What is the smallest screen size you design for

I'm looking to make my website responsive, however this is my first foray into responsive design, and I'm pretty clueless at the moment.

I have no idea what the current state of play is with regard to screen sizes for phones, hand-held gaming machines et al, so I'd appreciate some advice on what the smallest devices are that you currently design for (in pixels). I know that there are more variables to it than this but it would be a start.