Lightroom does not publish or export "preview" files. By default, the exported files are "full size". During the export you can set the size to be bigger or smaller if you choose. You also get to choose the amount of compression used in the exported JPGs, which can have an impact on the quality of the output.
What exactly do you mean by "publish", Bryce?
the publishing feature. Publish to Flickr etc. Are those the same quality as the original pictures or a different quality?