In my mind a 600' to 650' tower would be a good fit. Tall enough to be a focal point in the city but not so tall to get everyone worried about their views.
As for looks, I am partial to a conservative modern style, which combines the traditional rectangle with complimentary curvature, much like the Two International Finance Center in Hong Kong, but sof course about half that height.
For location, two blocks that seem perfect in my mind are: 1. The food cart block on Washington and 10th and 2. The Parking Garage with Tiffany's on 4th and Yamhill.
Both spots are centrally located and would place the new building between the US Bank and Wells Fargo buildings.