Tube diameter relates to tube stiffness. So how stiff do you want the frame to be. This relates to the rider's size and the frame's intended use (such as carrying 75lbs of camping loads or of riding centuries and wanting as smooth a ride as possible). For a first frame consider going luggless where possible. Far easier to end up with the geometry that you want (less lug bending). The crown and BB are the two joints where using socketed fittings makes more sense. Andy.