: Is there a "standard" rub rail size?

No. Use what every you have handy.

: . . . The book "The Kayak Shop" indicated 1/4" X 3/4".

This size is nearly standard for building canoes and kayaks, so if you are building a stripper you'll have lots of strips around that are already this size. So, I guess it makes sense to just grab a few and add them as rubrails. Whya re they this size? Well, if you want a hull that is 1/4 inch thick you make strips 1/4 inch thick. Cutting these strips from a common 1x6 or 1x8 board ( which is really only about 3/4 inches thick) give you the other dimension. No reason you couldn't rip them from a 2x4 and get strips that were 1 1/2 inches wide and 1/4 inch thick, or thicker.

The thicker the strip you use the more difficult it will be to bend it around the outside of the boat. you can go up to 3/4 inches thick, but you'll need lots of clamps to hold it on while the glue sets up, or you'll need to screw it in place, and that would call for a piece of wood or a chine inside the hull to screw into. 1/4 inch strips are very flexible.

hope this helps

PGJ