I assumed there should be an easy way to limit the size of a shared folder but I can't seem to find it? Is there some easy way to do it, preferably without 3rd party software?
I don't think there is a way to do this. You can have a workaround for this by creating a virtual drive of that size, or by creating user profile that has a quota.
If you are using windows server, here is a link for how to do it in that, but it is not possible in windows 7.