What should I do if the locker opens a drawer which is too small for my parcel?

If the drawer is too small don't close it, go back to the monitor and select "open an M size" for a Medium compartment or "open an L size" for a Large compartment.
After choosing the most suitable one, the locker will ask you to close the opened one and scan the barcode on the label again.
At this point, the larger compartment you chose will open! Remember: after you closed the compartment, confirm the operation on the monitor.