This method, directly pasted from Autodesk Knowledge Network would appear to be the solution you seek. Using a plane for the two dimensions you wish to scale should exclude the third from the changes.
Use this method to resize the objects only along a plane that you specify.
Click Home tab Modify3D Scale.
Select the objects and subobjects you want to scale using the following methods:
Press and hold Ctrl to select subobjects (faces, edges, and vertices).
Release Ctrl to select entire objects.
When you have selected all objects, press Enter.
The Scale gizmo is displayed at the center of the selected object or objects.
Do one of the following:
To scale along a plane: Hover the cursor over one of the bars found between each of the axes of the gizmo until it turns yellow. Click the yellow bar.
To scale uniformly: Hover the cursor over the triangular area closest to the center point of the gizmo until it turns yellow. Click the yellow area.
To scale along an axis: Hover the cursor over one of the axes of the gizmo until it turns yellow. Click the yellow axis.
To resize the selection, drag and release, or enter a scale factor while holding the mouse button.