The sleeve should cover your wrist and reach the beginning of your thumbs, while your cuffs should be tight enough to prevent them from slipping down your wrist.
If you choose to leave your button-down shirt untucked, it should hang just above your pants zipper (at back pocket level).
When wearing a jacket and extending your arms, the sleeves should land between a half-inch and one inch past the jacket. Anything longer is cause for tailoring or buying a smaller shirt.
And, by-all-means, if when you try on your shirt it bunches up between and across the button holes, it is too small! Choose another size. If you're in between sizes always round up.
