Liquid Technologies offer a large file editor in their product, its not free but it is available for a 30 day trial, so that should be plenty to accomplish what you have in mind. Their website claims it can open GB and TB files so again 6MB shouldnt be a prioblem. I havent used the feature myself so please do feedback if it works or not. More info here; http://www.liquid-technologies.com/Large-File-Editor.aspx
Notepad++ is free; I haven't found a file it can't open and 6 MB is certainly within its range. It offers syntax highlighting, collapse/expand tags for a variety of "languages" including xml.
Everything is theoretically impossible, until it is done. ~Robert A. Heinlein