That was an old Bill Cosby joke about building an Ark

Anyway,
Everything I've read points to the right length of a greenland paddle as an arm-span plus one cubit. A cubit is the distance from your elbow to your finger tips. Roughly, as high as you can reach. (Arm-span is about your height and most ppl's elbows reach the top of their head thus leaving a cubit to go).

Blade width should be as wide as you make it and still hook your fingers firmly around it in a 'C' shape. That way, you can still hold the blade in a storm stroke and various braces. Mine is pretty narrow becuase I made it from a 2X4 and still bites the water fine.

Loom should be a little wider than your sholders. I find myself half grabbing the loom and half the blade root.

As far as performance / speed. Once I learned how to use a greenland paddle right, I couldn't tell a difference from a euro blade. True, the first time I used my GL, I threw it back on the pier and grabbed my euro. Now, I like it a ton more and can go just as fast and hold the speeds just as long, if not longer. Plus, my arms aren't as tired on an all day paddle.

$.02

Don't give up on it just yet.
Ciao,
-David