You have a lot of freedom with specs such as neck width, neck taper (if any), neck thickness from nut to box.
You didn't say how many strings you plan to use, that obviously would make a difference in neck width, particularly at the nut. Also your preference for string spacing plays in to your choice of neck width.
For a first CBG, you may want to stick with a three-stringer. They are easy to learn to play and with amplification can sound amazingly good.
The default neck width for a 3-stringer is 1 1/2". That is a standard 1 x 2 from Home Depot. Oak makes a decent neck for this design. It is 3/4" thick which will resist bowing if you use reasonably gauged strings.
I prefer a neck that is tapered from the nut to the box, but there is no need for you to do that. Just round the neck so it feels comfortable in your hands.