Go with a too big a capital building for now. If you keep that as a separate object (?) you can always resize and replace. It will either work in its larger scale or you can have outer walls, a yard, garden or other landscaping between the rest of the city and the final building size.

Also if you insured major avenues all lead to the capital building from different sides of the city the capital building is further emphasized and kept from being lost in its surroundings.

GP