Welcome back Gary. Jo and Jason both offer good advice. Since strength of fasteners is usually of less concern in models if it were me I would probably just look at the various parts (or full size drawings at the new scale) and just see what looked best. If you are working in CAD, you can draw in the size you choose (head height, head diameter or measurement across flats, etc.), see how it looks there and then change up or down until you are happy with it. I usually keep a table of standard fastener dimensions, including the above items close to the computer when drawing.
Bill