Hi Patrick,
The easiest way I have come across is sit in your kayak in the water with a pole about the length you will need and paddle with it gently. Then measure the dry area. That is about the shaft length you need between the blades. Blades come at various lengths and breadths. Choose one you are happy with, short and wide or long and thin or long and wide depending on your style - usually long if you paddle low, shorter if you paddle high. Shaft length will vary somewhat from kayak to kayak as one will sit higher than another and the seat height will vary. A SOT will require a longer shaft than a SINK.
JohnK

: Shorter or longer?

: I use a 220 carisle and it bites hard and the shorter length is close
: in..does good.

: I have a 240 cm day tripper with a softer bite the stroke is longer and
: softer..also does good.

: The longer stroke means the paddle is in the water longer which i think is
: good for stability in chop.

: whats the ideal length? I am 6' tall. What about in a wider sit on top
: at 28" beam. Would you want a longer paddle for that?

: I'm looking for what is the general concensus then will try things out some
: more. I keep both of those on board.