I don't think you can do anything about the length of posts with overrides, although I could be wrong. Here are the FAQs about customization and overrides. You might also look through howto. I pretty much left my journal alone until I got a permanent account (and thus, use of the style system) so I'm really not that sure what can and can't be done.

I'm not sure quite what you mean by "change the dialogs". Could you give me an example?