Ad Manager……

I am having problems editing my Advertising Manager plugin. I wanted to change my ad size. I followed the correct steps, however, now I don’t have any ads on my home page, and have the original sized ads on my other pages. I have tried to delete and reactivate the plug in, but still nothing. Can anyone tell me what I have done wrong? In case you didn’t know, I’m a complete WordPress novice and learning as I go along. And I’m not the sharpest tool in the shed when it comes to computers…..so any help would be appreciated.
Julie