The Tiffen Soft Grade ND 3 provides a 1-stop light reduction (0.3 density) at its darkest point, tapering off gradually to clear glass.
Yes, this is a 4x5.65" filter, which is the industry standard for professional cinema matte boxes like the Arri LMB or Tilta MB-T12.
A soft edge has a wide, gradual transition between the ND and clear sections, making it easier to hide the transition line in uneven horizons compared to a hard edge.