First and foremost a coding screencast is about the code, and we need to make sure it looks great. There are a few aspects to this that help ensure that is the case.
720p is the target resolution. In pixel terms this is 1280x720. We've gotten the best results when we record at 2560x1440 in a HiDPI (pixel double) mode, giving an effective visible resolution of 1280x720, but extremely crisp. This resolution is achievable on 27" monitors and retina MBPs.