Use the Manual workflow pilot (red globe in the the upper left corner of the interface.
Go to the scan dimensions panel.
Select the expert settings (click on the gear wheel)
Place the red frame around the area you want scanned.
Check whether this gives you the linear dimensions you need, which you can read in the Expert dialog
If yes, scan,
If not, amend the dimensions in those boxes and watch that the red frame properly surrounds the area you want scanned.
Make sure you have selected the PPI resolution you want.