You can't prepare texture mappings in Blender that won't stretch textures in the exported OBJ file when you resize your imported piece.
But you could use 3 (or more) boxes instead of one in Sweet Home 3D, one for the 4 front/top/back/bottom sides, and 2 (or more) thin ones for the left and right sides. Once grouped, the textures on the boxes will behave as you expect it when you resize the group (see the file attached to this post).
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BeamWith3Boxes.sh3d
(66535 bytes)
(Download count: 131)
(Beam made of 3 boxes)----------------------------------------
Emmanuel Puybaret, Sweet Home 3D creator
