The term "Long Form" refers to the traditional form that is typically described as having 85, 103, or 108 movements, depending on how you count. This term is generally used in opposition to the "Short Form" taught by Cheng Man-ch'ing that is usually described as having 37 movements and takes typically less than half the time to perform as the "Long Form."
Both the terms "Long Form" and "Short Form" are widely recognized and informally used by many practitioners I know in the U.S.; however, this dichotomy is truly relevant only for a subset of Yang-Style practitioners, since Yang Style practitioners of different lineages now practice a wide variety of forms of various lengths and speeds and the term "short form" is closely associated only with the form taught by Cheng Man-ch'ing and his lineage.
Also, as far as I am aware, Cheng Man-ch'ing did not particularly care for the term "Short Form" and used other words to distinguish his form. As for the Association, we have four different forms which I generally hear called the "Traditional Form" or the 103, the "Demonstration Form" or the 49, the 16, and the 13. Some might informally call the 103 the "Long Form"; however, the term "Short Form" is not used to refer to any of our forms.
In no case does "long" or "short" refer to the length of the stance.
I hope this helps.