Men buy jewelry. Men wear jewelry. Here's a chart of 'standard' lengths of various pieces of jewelry for men. Use it as a guide to help you select jewelry for a man or to make it yourself. Remember, just as a 16" necklace won't fit all women, an 18" necklace won't fit all men! I would say that the standard sizes are for an average-size, relatively young/trim man. You can ask a man his shirt collar size (you know he'll want his necklace longer than this length) or whether he wears a standard-length wristband for his watch or an extra-long length. Men tend to prefer bracelets that are close-fitting (not tight, but not slipping onto his hand or up his arm). Precious metals, gemstones, leather, glass, and wood are all good options for materials. The design should suit the man's personal style. Make sure any clasps are durable and easy for larger fingers to open/close. If you're looking for ideas for men's jewelry, my Jewelry for Men article may be of use to you.
Making Jewelry for Men - Lengths and Sizes 
Men's Jewelry Lengths
