What size of a pipe will it hold? I want to use these for holders for Posts/handrails on a dock. I want to put posts up and a chain I can remove when needed instead of wooden handrails.

The standard pipe sleeve fits a 1 5/8" diameter pipe and is 3 inches tall.

What is the horizontal and vertical distances in inches between the centers of the square holes used to mount?

The vertical distance is 2” on Center and the horizontal distance is 3 ½” on Center between the holes.

What size carriage bolt does it take to fasten it to a 2" by 12"?

We recommend 3" hot dipped galvanized carriage bolts with, flat washers, lock washers, and nuts.  PlayStar PS 1135 Dock Frame Hardware.

What is size and length of bolt that holds pipe?

That bolt is a 3/8" x 1" Hot dipped galvanized hex bolt.