I want to know how to export video with a resolution larger than 1080, ie 2K and 4k. I cannot find anyway to do this as all the presets in CS5.5 only allow upto 1920 x 1080. Please help!!

You need to create your own preset for QuickTime, if this is what you want to use. DPX does have proper presets already. First choose the container from the list (not all will support higher res though), then select the codec in the video tab (if necessary), and choose your resolution there as well. For example you can choose QuickTime, JPEG2000, and any resolution you want. Then save your preset or export the movie and you're ready to go.