I decided to make a backgammon board as a gift and wound up making two sets. I didn't find any good plans online so I'm posting them here in case others want to follow them. I made it at Techshop and got a lot of great advice from the folks there.
The first thing to decide is the size of the pieces that you want. The two sets I made were 1 1/4" and 1 1/8".
From there, you can figure out the height of the points - I used points 5x as tall as they are wide so that five pieces fit nicely on each point.
Many sets have extra room on the edges of the board to store the pieces as they are taken off. It's up to you if you want this - I made one with and one without. If you include it, you'll need to add extra room for the width of one piece plus another 1/8" or so for the separator.
The last thing to decide is the width of the frame. I used about half the width of the pieces.
Using all that you should now know the dimensions of your playing surface and the inner and outer dimensions of the two frames.