keep 4 piece on door fixed size, just change the center piece of door

If I understand correctly, you probably want to edit the .obj with another software. The only way I know to do this with SH3D is if the centre piece has a different colour in the SH3D file (that is a different material in the .mtl file. Then you can set first the centre piece to invisible and export the rest. Then set the rest to invisible, make the door visible, and export. Then you have two different element you can import individually, scale and fit together with SH3D.

But, editing the .obj file with other software is probably easier.

ok