I assume the center stone is not bigger than 0.25 ct., so the sides should be around 0.15 or 0.10.
I have a 0.5 ct. solitaire, and my finger size is 5, so I have a good basis of comparison, and the center stone is definitely smaller in the ring on the picture.
I would take down measurements of the diameter of each stone and then use one of those "actual size of diamond" type of charts that give you the carat weight and corresponding mm measurement to guesstimate.