ENGAGEMENT DIAMOND RING - what size?

[AOX81]

To tell you the truth I'm not sure how big my diamond is. My band is white gold, the diamond in the middle is a round cut, and it has two smaller diamonds on the side.

I would have truly preferred just a band because my ring gets caught on EVERYTHING!!! I told that to my husband and he kind of got upset. I tried to explain to him that I loved the ring that he picked out for me but I'm just not very careful. I'm surprised I haven't broke a prong on it yet!