What was the largest instrument of the violin family ever made and who was its maker?

The largest double basses are around six feet tall, Paul; but I suspect you are referring to something uniquely constructed as a 'one-off', of which I am unaware.

An almost 16 foot tall double bass was made by Paul de Wit as part of the celebrations for the Cincinnati Music Festival in 1889.

Well, Jersey Tomater, I think your answer may have overridden the answer I was hoping for. I wasn't aware of a 16' bass.

The one I was referring to was built by a French violin maker named JEAN-BAPTISTE VUILLAUME who lived from 1798 to 1875. He built a 12-foot-tall three string behemoth called the octobass in 1849 that had to be tuned with the help of an assistant.
That was supposedly the largest member of the string family created at that particular time.

Thanks for your answer though. It seems I learn something new every day.

Peace to you!

Paul