Sounds like you are used to touring paddle lengths. While a 204cm paddle could very well be ideal for you, its not like you need to get a custom paddle to enjoy whitewater. You will get used to whatever you paddle with. I'd suggest paddling with the 200cm paddle for awhile, unless the cost of a longer paddle (or a custom paddle) is not an issue. Also, I think blade size is more important than a 4cm change in length.
I am 6'3" with a +4 inch "ape index". That is: my "wingspan" (fingertip to fingertip) is 4 inches longer than my height. I love my 197cm old AT ATX. I prefer the larger blades on the ATX (and also the softer flex and ergonomic grips, which are easier on my wrists and elbows). I have not felt that much difference between paddles (ATs with similar blades) ranging from 196 to 200, but smaller blade size feels like a huge difference in harder whitewater, with more strokes needed, and a somewhat less effective brace. the ATX blades are 748 sq cm. My AT4 River glass paddles are stated as 760 sq cm blades. If you get out to Utah, you could try my 200cm AT4 River glass paddle. ATs website says it has 760 sq cm blades, which I find to be a good, large size. Its not easy to calculate the surface area of blades, but I believe the ATs are larger than Werners fwiw. The Shogun says:
Surface Area 728sq cm
Werner Paddles :: Our Paddles :: Whitewater:: Performance Core:: Sho-Gun
AT Paddles / Whitewater / F-Core-Series / ATX Flexi
748 sq cm
I see that Werners standard sizes (in all models, not just the high end paddles) go to 203cm in a bent shaft, and 206 cm in a straight shaft. I think AT could make you a mid-range, long paddle also, without breaking the bank. So you don't have to spend a lot on a custom paddle to get a 204cm paddle.
Here is a Premium Powerhouse for example,
Straight Shaft 185-206cm
Neutral Bent Shaft 188-203cm
Straight Shaft $240
Neutral Bent Shaft $315
Note that larger blades can put more force on your shoulder joints if you have bad technique. Keep those elbows low to avoid a shoulder dislocation. It doesn't matter how strong you are; the river is stronger.
I prefer the AT ergonomic shafts; going to a straight shaft Werner breakdown was a big transition (figured I should practice with it, in case I needed to use it). Going to an unindexed (round) straight shaft is an even tougher transition.
I know some larger guys who break alot of paddles. So you may want to go with the a tougher paddle (instead of the lightest), and/or be real sure about the warranty from wherever you buy. I have to say that H2O did NOT stand behind their paddle; I broke one without rock contact, trying to roll in a hole; in the first couple months I had it; very disappointed in H2O since I did like the grips and performance. I managed to hold on to the longer half of the paddle too. It helps to have the longer half of the paddle if you break a Werner and want to warranty it.
Werner from what I hear has great warranty and customer service; AT also. I'm excited to try a Core paddle; in the shop they have a nice flex, and the grips are a bit farther apart, so some paddlers are going down a size.
CORE Whitewater Paddles
Since you mentioned that you are new to whitewater, be aware that paddles can easily get lost in a swim, especially on IV and V runs, and in big spring runoff. I know an expert paddler who lost his paddle this spring. He was able to go to an old paddle with smaller worn-down blades, and still paddle hard runs. But he had to adjust to taking more paddle strokes.