This article has some great thoughts on the quote, what it means, how you can measure better.
If you measure the size of the church, its the number of people in the church you will work on improving.
(What happens when people switch to another church? When people stop going to church? Do they get removed from the church rolls? Is the church eager to start new churches?) If you measure the number of churches, its the number of churches (typically building, or houses) you’ll work on adding to.
(Are the church buildings maintained even if they have no people in them?) If you measure the number of missionaries, you’ll get more missionaries - but many will be working in easier to reach places.
(We send thousands of workers! But most go to the same places.) If you measure unreached individuals, you’ll (hopefully) start recruiting people to focus on them (evangelists), but they’ll focus on the places with the largest number of unevangelized.
(This can be good and bad, in different ways.) If you measure unreached places & peoples without workers, you’ll start recruiting workers to go to each individual place/people.
(But make sure you look at the ratio of workers to unreached peoples--one team for millions is not enough.)