Bill, Many thanks for the diagrams of the Whyte notation.
John, the manufacturer will give the length, but perhaps more important, the recommended minimum curve for each engine in its documentation. My MTH engines have this information right on the label on the box, and online. Other manufacturers do the same. It is something to keep in mind with passenger cars and long modern freight cars as well.
Here is an example using my largest engine: https://mthtrains.com/20-3166-1 Now, I bought this engine used for a little more than than half the original MSRP from a member of this Forum. All but one of my engines were bought used. The new one was an inexpensive small diesel.