This is really up to you and the devices you want to support -- a width of 480 in addition to the one you have might be enough. Just test font sizes on those smaller devices to make sure it's something you're comfortable with.
Here's one approach to layouts that might be helpful by @michelangelo: How many layouts do i need to make? - #7 by michelangelo
If you have pinning on the left, top, and right, and scaling (on the width dimension) it will scale up and down with your device's width.