Claw sizes are listed for most fits, you can look up the claw size as well as other fit notes here .
If the claw size isn't listed here is how to determine the correct claw to use. The RailGrab should have three points of contact on the rail.
For rails with a height of less than 1 1/16th use the Small Claw
For rails with a height of between 1 1/16th to 1 3/4 use the Med Claw
For rails with a height greater than 1 3/4 or odd shapes use the Large Claw
Please refer to pages 2-3 of the instructions found here