It sounds like you might be new to woodworking. I would suggest trying to find a local woodworker through a Craigslist ad or through your local woodworking tool store that would be willing to work through some of the basics. I think you will find woodworkers to be a generous lot and are always eager to pass on a little information. Generally speaking, glued up sections are usually done in equal (roughly, visually) parts for appearance purposes. The general practice though is to make the pieces a little oversized so that the finished pieces can be brought to finished dimensions and squared up as needed. Some folks will use the board dimensions in order to incorporate joinery details such as mortises.