I would think that a mid-sized coastal empire with lots of Citizens and good trade goods should be pretty viable. A high Citizen ratio means better tech and more commerce income, and it should be easy to keep happiness high and optimize building placement for good output from a modest pop base. Mercenaries help make up for lower manpower.
Obviously you'd still be less powerful overall than a major empire, but you could punch above your weight.
Obviously you'd still be less powerful overall than a major empire, but you could punch above your weight.