Our cuff sizes are approximately:

Small - 5 1/4 inches in length, a gap of 3/4 inches and 2 1/4 inches in diameter wide - fitting wrist 5 1/2 to 6 inches

Medium - 5 3/4 inches in length, a gap of 3/4 inches and 2 1/2 inches wide - fitting wrist 6 to 6 1/2 inches

Large - 6 1/4 inches in length, a gap of 3/4 inches and 2 3/4 inches wide - fitting wrist 6 1/2 to 7 inches

So if your wrist measures 6 inches in circumference, you should select a medium sized cuff.  This will allow some movement and the gap can be squeezed in to allow a tighter fit.

Bangle sizing is different than cuff sizing since the bangle must be fitted to slide over you hand as you put it on. 

To measure, fold you thumb across your palm towards your pinky finger.  Use a measuring tape to measure the widest point across your hand.  This is your hand circumference and is the size bangle you should order.