Here are my specs. They were done by Bucks County Soapstone, a regional company that only works with soapstone so they know their stuff.

My sink is 30 x 18 inch depth so I only have 5 runnels. They are the thickness of my index finger. You can see how they taper from thin and shallow and get deeper and larger to encourage drainage. I have an iron dishrack from Ballard Designs which fits perfectly over the runnels. That is how I chose 15 inches for the length. 18 inches may have been ideal in retrospect. The grooves are evenly spaced starting from about 3.75 inches from the front of my sink.

5 grooves
15 inches long
1/2 inch wide
1/4 inch deep tapering to 3/8 inch deep so the water drains toward the sink. Here is a picture.

My sink is roughly 27 x 18 "
I have 7 runnels about 1 3/4 " apart 18 " long.
The runnels begin and end about 5 " from front of counter as well as back.
They are 3/8" wide as well as deep.

I don't use any kind of a dishrack.
If I need to rest a tall baking sheet against the backsplash the rear runnel is close enough to the wall to hold it upright.

Hope this helps.
AND I LOVE my runnels!

Be sure they are 1/2" wide. Mine are less than that which makes them a PITA to clean and oil.

After reading the last post I went back and remeasured the width.
I was wrong.
Mine are 1/2 " wide and 3/8 " deep at the deepest, sink edge.
No problem keeping them clean.

Hope I am not to late to help. Typically we install our runnels/drainboard 1/2" wide 3/8" deep at sink and 1/8" at other end to provide the necessary pitch for water to drain. Our, what we call colonial look is 14" at the center runnel. We found this was a good length. http://www.gardenstatesoapstone.com/gallery-runnels-drainboards.php
However you can customize your length to what ever you feel comfortable with. Hope this helps.
Goodluck