Automatic Sizing of Planes and Axes
Planes and axes that you create are automatically sized to either the geometry on which they are created, or to the bounding box of the model geometry. As the geometry changes size, the planes and axes update accordingly.
You can override the automatic sizing by manually changing the plane or axis size, which disables future autosizing for that entity. You can re-enable autosizing by selecting Autosize from the shortcut menu.
The created plane is 5% larger than the geometry on which the plane is created, or 5% larger than the bounding box. This helps reduce selection problems when planes are created directly on faces or from orthogonal geometry.
To see an example of automatic sizing of planes: