I recently rooted one cutting from Ruby Supreme guava. I had 5 cuttings in different diameters and only one, a pencil size, rooted.

Nice. how did to you do it?
Added a few drops of rooting hormone to the water. Let the cuttings to stay in this solution for a couple of days, watching the water, then replaced with fresh water and so on every other day. The cuttings were on a sunny window sill for a month or so.
You're smart to change the water with the hormone. The hormone that promotes root initiation will also inhibit root growth. More is not better in this case.
I did something similar to wax jambu. Green wood cutting, remove all leaves except top pair, treat with hormone and root in water in bright indirect light, change water every few days to wash away bacteria growth. Rooted in 3 weeks for me the first time. I find that cutting half of a leaf will tend to make the leaf fall off faster. Now I just leave the top pair with no cutting them shorter.