There are three key dimensions to an effective
Overall length and loom length are the most crucial to paddle performance. The width and thickness of the loom is determined by your hand size, and is less critical. A nominal width of approximately 1.5" and a thickness of 1" works well for most people. These can be adjusted for those with either very large, or very small hands.
The following sizing guidelines will yield starting points for me to determine how to make your paddle. It is important that you call me after taking the measurements so we can review your individual needs and preferences. This is especially important if you are less than 5’-6” tall, or have a kayak wider than 22”. Determining Overall Paddle Length:
Method 1: 
Stretch your arms outward and parallel to the floor. Keep your elbows and fingers straight. Have someone measure the distance between your longest finger tips and write this down. This is your Horizontal Reach.

Place the elbow of your dominant arm on a table or bench top with your forearm and hand pointing straight up. Have someone measure the distance from the table top to the second joint of your middle finger and write this down. Call this your Cupit.
Add the two measurements together.
Method 2:

Stand in your bare feet on a hard floor next to a wall and extend your dominant arm straight above your head.
Have someone measure the distance from the floor to the curl in your elevated hand along your finger tips. This is your Vertical Reach.
Since Paddle use is a function of your upper body, Method 1 is usually a better predictor of performance. Method 2 can be used as a cross check. Be prepared with both measurements when you call so we can determine what will be best for you.
Determining Loom Length:
Some people say the loom should be the same width as your kayak. I think this results in a longer loom than most people will find comfortable. However, the above measurement can be adjusted for people with narrow shoulders and/or wider boats.
Loom Width and Thickness:
Blade Width:
The blade width on my paddles will usually be 3.5” at the tip. This is a function of lumber dimension, and works well for most people. I can make the blades narrower if you wish. Please mention that when we talk.
Shoulder Style:
I make my paddles in two shoulder styles: 
Full taper to the loom, or:

Modified step at the loom. Here the blades taper to half their overall width approximately 1.5” from the loom, and then taper to the loom.
Paddles with fully tapered blades have less surface area. They require less effort per stroke, but also have less power. They can be more comfortable to grasp, especially when using the slide stroke, and are generally lighter in weight .
Paddles with a modified step at the loom have more surface area, and thus afford more power to each stroke. The modified step (as apposed to a full step), still provides a smooth area to grasp, and sacrifices little comfort.
The choice is purely a matter of personal preference.
A photo summary follows below.