Herm Sprenger bit sizing problem

Hi,

I purchased a Herm Sprenger Dynamic RS WH Ultra D-ring snaffle yesterday, which the store (Dover) said was 5 1/2 inches. Her old bit was 5 1/2 inches and fit perfectly, I loved the HS, but it seemed too big, and it measures just over 5 1/2. I read they are truly measured in millimeters and there is no exact conversion to inches-in this case should I go down a size? It seems like the difference moving down might be too much, but I also don't want the bit too large. Suggestions?