This value can be used to get or to set the scale of the currently assigned path for the instance (as set by the function path_start()) with a default value of 1. This is a scalar value, so 1 is a scale of 1:1, while setting it to 2, for example, will be double the scale and setting it to 0.5 would be halving the scale.
path_scale;
path_scale = 1 + random(2);
The above code will set the current path's scale to a random size between 1 (its original size) and 3 (3 times its original size).