The classic size of pearls used in bracelets is 5mm - 8mm, and is appropriate for most any occasion. Larger sized pearls, such as 10mm, should be reserved for special events.
White Akoya pearl bracelets are perfect for any occasion. They add elegance to the wearer, and look great with either a dress or jeans.
For an important occasion such as a birthday or anniversary, give her a large high quality Akoya or Tahitian pearl bracelet. You may also formalize the gift with a matching pearl necklace and earrings. For a more unique look, consider a double strand pearl bracelet with stylish clasps. If she is more fashionable, multicolor freshwater pearl bracelets are a good choice.