Order the larger size. A bridesmaid dress is typically ordered using a person's biggest measurements, so your friends will likely buy dresses at least a size larger than they normally wear. If a friend wants to buy a size smaller, let her know it's much easier to alter a bigger dress, and if the dress ends up being too small, she'll likely have to order a new one and you'd hate for that to happen.
Pick out the right color. There are several foolproof ways to pick the perfect color. One of the easiest methods is to play off of the color schemes of the venue and the season. If you're still at a loss for exactly which hue to choose, go for a general color, like green, and let your bridesmaids pick their favorite shades from light to dark for a trendy, mismatched look.
Set a budget in advance. In general, the bridesmaids would pay for their dresses, so it would be not too expensive. There are so many kinds of bridesmaid dresses under $100 online you may find. If you don't want your maids cost their arms and legs, help them find a reliable website and place the order.