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.
Thanks for the replies. Yes, I did mean a library Block created as a 1 by 4, 80 inches long. I thought I might be missing a way to change the length of the Block without Exploding it and then editing as lines. I also tried the Modify >Properties approach but when I set X scale to .5, Y became blanked out and I was unable to leave Y at 1 so both X and Y were scaled to half size.
Sorry. I mis-spoke above when I commented on trying Modify > Properties. I was trying Modify > Attributes and with ISO checked when I could not independently control the X and Y inputs. I will try the Modify > Properties as suggested. Thanks.