I use rods from a short 36'' for working bridges with very light jigs (1/100) up to rods 12' for spider rigging. If I'm casting I like a 5-6'' rod as they seems to be about the right size for me. I also fly fish from my kayak and have used up to a 9' flyrod but use a 7' 3wt on most trips when fly fishing.
Kayaks don't have to limit you on rod size.