These are just for the breeches o_O
Above is the J.P. Ryan fall front breeches pattern.
I figure that starting with the breeches will be best, so I can more easily determine the length of both frock coat and waistcoat.
The waistcoat needs to be long enough at the center front to cover the waistband closures on the breeches, and the coat's front corners need to hit right above the breeches knee band. Perhaps a little higher, but then as now, it's entirely up to the wearer.
Through the 18th c. longer coat lengths were used for business and dress occasions, while the shorter lengths (as short as mid thigh) were used for sport; hunting, etc.
My gent is rather tall, so we will probably stick with the longer option.