What is the good ganularity of services ? Well I like to reformulate this question in what is the manageable granularity of services. How many services methods or interfaces can we manage in an Enterprise. If we take a decomposition like APQC Process Classification Framework at the task level which is the 4th level of decomposition, we get aroun 1,500 tasks for the cross industry elements. Each task would have several interfaces or methods. Looking at other decompositons like IBM's Component Business Modeling which is a two level decomposition or Telemanagement eTOM Business PRocess Framework we get an average of 7 to 10 elements at each level of decomposition. This would lead to a potential of between 10 thousand and 100 thousand interfaces at a level 5 of decomposition, which I think everyone would agree is not manageable.
The implication of that simple math is that a manageable granularity is between a level 3 and 4 of decomposition, and if you end up with finer interfaces, we need to consolidate into more variable paylod interfaces at a higher level. It also implies that a decomposition exists in the enterprise otherwise there is no way to find what is the decomposition level of a service.
In addition to the above mentionned CBM, APQC and eTOM, a method to find a tree decomposition from process variations is described by the article from MIT Sloan School of management A Coordination-Theoretic Approach to Understanding Process Differences.