In API v1.1, the Twitter docs now tell you to “modify the URL” rather than make another request to get the URL for a different image size. So basically, like @pete_klein said, take the profile_image_url from a previous API call (like get friends) and do a string replacement for “normal” to “bigger” or “normal” to “” to get larger profile images.
It feels a little sneaky, but so long as the conventions don’t change it’s one less API call for Twitter’s servers to respond to, and one less API call for your client to make.