You need a long chain that will reach from the pants waist to an out-held hand. That would be about 36 inches.
Two Pacific Electric (Los Angeles area interurban) trainmen developed and initially manufactured the spring-loaded key-back to secure their switch keys to a chain that would not get in their way. They did not want to drop (and loose) their keys on a dark, rainy night into a puddle of water and they did not want a long chain that would be in their way.
The Pacific Electric company magazine had a write-up in the late 1940s about the product that was made by the two men and their wives.
See this history page at the Key-Bak website:
Thanks for sharing the Key-Bak history. I’ve pasted the link content here because sometimes web page links “break” later on.