What effect will the population of a province have? I assume it affects tax, manpower, but can they rebel? Can they defect? Can they like or hate you?
There is no population size for a province but every province has a tax value and a levy size that can be effected by modifiers and buildings. They can't defect but they can rebel - and if the rebels siege it down they own it, which means that anyone else can go there and siege it back. There might be a dev diary with more information on rebels so I'll leave it at that.