If we wanted to compare muscular endurance between a very large person and a very small person, what method would be the best method to use?

I would be best to use the fixed percentage of RM Method

This method could show could evaluate their muscle endurance by determining the Repitition maximum that they could achieve when they do a specific required exercise such as : bench press, shoulder press, or pull ups

Usually , research put 7  - 10 repetition maximum as their standards

Inaccuracy is a disadvantage