There is a great resource for following the height of buildings in the Downtown Bellevue area! A wiki on Wikipedia has kept up with the current building heights, as well as what the projected building heights in the area will be.
Currently One Lincoln Tower is the tallest building coming in at 450ft & 42 floors. Right across the way is Lincoln Square, which is the second tallest building in Bellevue at 412ft.
Looking toward the future Bellevue Towers is projected to match the height of One Lincoln Tower at 450ft tall, sharing the title of tallest building in Bellevue.
The site is a great resource for not only the tallest buildings, but projects under construction, approved, and proposed.