Many mens silver chains are quite thick which gives a rugged impression and exudes a look of solidness which the owner finds important. With this in mind a silver necklace for a man should be at least 5 or 6mm across unless it is worn to display a pendant which will allow for a finer chain than that for a necklace worn solo.
Bracelets for men should be thicker again, 7 or 8mm will look masculine, less than that is probably more suited to the fairer sex. Heavy bolt ring clasps are a good way to dress up a silver chain and still remain manly in appearance.
The right jewellery will be appreciated and happily worn by many men, but chains that are too fine are likely to be put away in a drawer and never seen again.