Two solutions come to my mind:
- WonderCMS is the smallest CMS you can imagine (it's only a 11KB download). Its concept is similar to what some others have posted: Just a login, a textarea and some minimal extra settings. You can also add pages and build a simple navigation, but it's easy to prevent that.
- OneFileCMS is not really a CMS (but even smaller than WonderCMS: 6KB). It's an online file manager with a simple text editor, which consists of one single PHP file. That way you can build your "template" and include one file in a subfolder which contains the editable content. Just put the OneFileCMS into that folder and it can handle the login and editing.
