As an alternative, you can use Skeletal tracking with the Kinect for Windows sensor. This problem will get reduced to distance computation between the ankle/Knee joints(3D points), whichever fits your definition for distance.
Note: Kinect for Windows is supported in MATLAB through Image Acquisition Toolbox, R2013a onwards.
No. You cannot determine the distance between legs of someone in a hoop skirt with a sufficiently long train.
How is "distance between legs" mathematically defined? Legs consists of at least 3 rigid segments (simulating the 26 bones of the feet seems to be over-doing here) with a spatial extent. While the distnace between the hip-joint centers is (almost) constant, the "distance" between the lower leg segments or the feet needs a meaningful definition before we can discuss, if this can be determined by a 2D-projected movie.