I am currently leading a team of 7 (including myself) and we currently operate a Monolith Rails application with several small to medium microservices.

Our philosophy is essentially, is what we need part of the core functionality of our application, and does it rely on other core functionality. If so then it goes in the monolith. But if it is independent of the other core features then we build it as a standalone service.

We started moving to microservices ~2 years ago when our team was only 2 (me and 1 other). It was much more difficult to maintain at the time, but the flexibility in being able to rewrite certain things when necessary was greatly beneficial. Things like automating our build and great documentation for how things interconnect has alleviated alot of the trouble we had running our microservices.