microservices: when to break them up, and when to keep them together

ive been seeing a lot of talk about microservices lately, and its great, but im struggling with the right size. when you start a project, its usually one service. then it grows, and youre tempted to split it. but how do you know when a split is truly beneficial versus just adding complexity? what are your heuristics for deciding when to break apart a monolith or when to keep a few related functionalities within a single service