CREATING a file limit for uploads….?

Hi I run three installs of wp on my domain (two of them are subdomains) I only have file uploading via the dashboard allowed on one but the person who uses it doesn’t really know a lot about image file sizes and keeps uploading files 1meg+ I wouldn’t mind… but it’s eating my bandwidth! LOL

Is there ANY way I can limit the file size upload on just one of the installs?

And yes, disallowing uploads is an option but it will not help my relations with my future sister in law.