My first reading was that you mean a block of wood you have drawn with simple line entities.
Then I would make a parallel line from on side with distance 40 and trim the two long lines there.
If the block is a rectangle you have to explode it first.
When block means a library block, then simply use the Modify -> Properties tool.
Set the X or Y scale to 0.5 and you are done.
Armin
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