The default clipped blade length is 28mm from the top of the first wire to the tip.
Leaving the blade longer than 28mm widens the shape of the reed and weakens the lever action created by the wires and internal bevel, requiring a rounder tube to prevent the tip opening from collapsing. This may cancel out whatever benefits a wider shape provides.
Clipping shorter than 28mm narrows the shape of the reed and increases the amount of cane available to scrape, therefore making the reed stronger and sharper. However, this is problematic and should be used as a last resort. The thumbnail pattern and tip bevel must be recreated as the clip distorts the existing balance.