Hi Ben.
We have a medium frame Cannondale about 6 years old and it is to small for us (we are both around 5' 10" ) but was available.
I ride with a Blind group as a captain and we have the large and extra large cannondales for group rides.
Both the larger bikes are a better fit and as a bonus I don't have my toes hitting the tyre when on sharp turns,
Sadly when we bought our cannondale it was the bigest available and has been verry kind to us and we forgive the toe clash.
But I always option for thr Large or EXL cannondale when captain for the vision impaired as it is one thing I can do without
The only time a medium or samll frame bike is chosen is for a shorter captain with a shorter stoker.
BUt the best recomendation is ride avery bike you can and find the most comfortable and stable bike that makes you happy.
Hope this is helpfull