The definitional size I've read and heard is that you team could (with the benefit of hindsight) be able to reimplement a microservice in 2 weeks. That sounds fairly extreme but a month seems within reason to me.
The other key difference between microservices and other architectures is that each microservice should do its primary function (temporarily) without hard dependencies, which basically means having a copy of the data that's needed. Service Oriented Architecture doesn't have this as one of its properties which is why I think of it as a mildly distributed monolith. "Distributed monolith" is the worst thing you could call a set of microservices--all the pain without the gains.
The other key difference between microservices and other architectures is that each microservice should do its primary function (temporarily) without hard dependencies, which basically means having a copy of the data that's needed. Service Oriented Architecture doesn't have this as one of its properties which is why I think of it as a mildly distributed monolith. "Distributed monolith" is the worst thing you could call a set of microservices--all the pain without the gains.