One thing to keep in mind with Firewire boards vs. traditional surfboards is they have a number shapes with lots of volume that are designed to be ridden shorter (much shorter in some cases) than your standard surfboard. For instance the Hellfire's are considered a "performance hybrid" and the conversation should turn more towards volume than length. Example, I used to never surf anything smaller than a 6'0" tri through years of competing at the pro/am level. However the 5'6" Hellfire has enough volume to easily allow me to paddle it in overhead surf. It's a big shift in how you think about boards and what you want to use them for. But once you know your number in terms of volume it opens up new doors for boards regardless of length. The general rule for the kite specific line that works for most but still depends on skill level is:
Under 185lbs/81kg
5'6" Hellfire 24.5l (light wind/everyday board)
5'8" KDM dominator 31l (light wind)
5'11" KP1 ~23.2l (high performance)
6'2" KFE 28.5l (larger surf)
Over 185lbs/81kg
5'8" KDM dominator 31l (everyday board)
5'10" KHF Hellfire 29.4l (light wind/everyday board)
5'11" KP1 ~23.2l (high wind)
6'0" KQF 28.5l (light wind/everday board)
6'2" KFE 28.5l (high wind/everyday board)
And all of these models except for the KP1 and KFE give you the flexibility of a 5 fin box setup allowing you to configure the board as a quad or a tri. This adds another variable as typically quads will go upwind better, are faster down the line and tri's will turn tighter in the pocket but may require a tad more kite. At the end of the day you have a lot of options even within the kite lineup which are constructed extremely well for kite use. Or you can delve into the myriad of options on the surf specific lineup of boards they offer but be realistic with expecations on longevity of the board if you are jumping or really hard on the boards. The FST build holds up a little better than rapifire for kiting on.
Hope this helps...sckitesurf