In wushu competitions there are several taiji forms, such as Sun-style 73 and the 42 combined set, but is there a competition form for Xing Yi?
Yes.
How long is it, and which branch is it derived from? Shang-style or a blend? I'm a pure Taiji practioner, so forgive me if my question is a little strange.
If the competition form for xingyi is the one also known as the "comprehensive xingyi form", then you can see it in Andrea Falks translation of Li tianji's book "The Skill of Xingyiquan".
Based on my quick look at the form in the book it appears to be kind of "stock-generic" Hebei xingyi.
The set looks interesting, it contains 77 moves and is supposed to include the 5 forms and all the 12 animals. There are 6 lines in the set (i.e. you go up and down the line 6 times).
All that information simply comes from Andrea's translation. I have never seen the form in "real life".
take care,
Brian