Crossbows are a whole different breed. My arrows fly best when endowed with high FOC, much higher than for a vert. The length of the arrow should not matter that much given the speed of your bow but get the weight forward.
In my 355 fps bow, I'm using 20'' Gold Tip Lazer II's, with 110 gr brass inserts and 2" Blazer vanes, plus 125 gr heads, for an overall weight of 425 and an FOC over 20%. For serious work I use 125 Slick Trick Mags.