Autosaves should be hundreds of kilobytes at most, unless you were playing a mod with huge changes and maps. However, in any case, Autosaves are only for you to use to restart a game from the point that the Autosave was made. So, if you never plan to reload that game from that Autosave, there is no reason not to delete it. I routinely delete all Autosaves when I finish a game.