The development firm taking control of the former Filene’s property in downtown Boston said it will build a tower of up to 600 feet, rivaling the tallest buildings in the district and creating a new magnet for shoppers, diners, and residents. A building of that height would be akin to the Federal Reserve building or the nearby tower as One Boston Place. It would be taller and more slender than the previous structure proposed for the site, a 39-story building that stalled in the early stages of construction.