The Meta title is best when its about 65 characters (meta title tag recommended characters length) including white spaces and should be keyword rich.
However, keep in mind, the meta title is also seen as the link on Google search results. So it needs to be attractive and contain enough information to entice a Google search engine user to click on the link.
As for what would constitute spamming, probably using the main keywords more than once or twice could be seen as stuffing considering the limited space.
What's important is that you include your most important keyword/phrase as close to the start of your keyword title as possible.