Welcome to the forum.
Independently of the help provided by chrisb, I thought to reply to one of your earlier questions:
Fon wrote:I think I have problem with boundary box.
The boundary box cannot be edited. It is calculated automatically by FreeCAD and its only purpose is to give a visual hint of an object's size.
Alternatively to the method of retracing the shape, you could also convert the imported mesh to solids. But to do that, you need to split the mesh into its separate mesh components with Arch SplitMesh
from the Architecture --> Utilities menu of the Arch workbench. This will separate the Cat mesh into 5 meshes.
After that, you can convert the meshes to solid as explained in this tutorial: https://www.freecadweb.org/wiki/Import_from_STL_or_OBJ#Conversion_mesh_to_solid
Once you have 5 solids, it should be easier to extract 2D geometry to extrude to a different length.