I look to this section of the specification.
A resized ad must position itself such that the entire close event region appears onscreen. If
the container/SDK detects that a request to resize will result in the close event region being
offscreen, the container/SDK should return an error, and ignore the resize (e.g., leave the ad
in its current state).
If I understand your question correctly, the resized ad could set negative values, but if that results in the close region being offscreen then the request to mraid.resize() should trigger an error.