Offline Gmail has been updated to version 0.2. As with all Google Labs features it isn’t rolled out to all users yet but will show up soon. Once it’s out though you will be able to specify a maximum size for the downloaded attachments. If you don’t need to access attachments when you’re offline, you can disable the download of attachments.
A user of the Gmail Group has a workaround for those who don’t have the new version. “It seems to me that for existing users it’ll appear only when you disable and then again enable Gmail offline. And you won’t have to download your emails again; you can always choose them not to be removed while disabling Gmail offline.” To basically, disable Gears for Gmail, re-enable it once you have v0.2, and you’ll get the new enhancement.