While there is no definitive answer and it really depends what your budget will allow, a 0.70 carat diamond is a popular choice for a centre stone because it looks just the right size - generous and eye-catching. Diamonds of 1 carat or more require a much bigger investment and make a very impressive statement. Conversely, even rings that are smaller than half a carat can be very precious and sparkly, particularly if you choose a round brilliant cut diamond.