Blind sizes are always measured by the millimetre (mm). Standard size blinds are for Venetian or Vertical and Roller Blind.
There are two standard curtain pair widths, 210cm & 280cm. The two standard widths refer to the approximate width of the combined width of the pair, after you have pulled the strings on the headers to achieve the desired gather. To help when ordering, just enter the overall width in cm of the opening that you wish to cover and the system will automatically select the correct curtain pair width for you.