The Seattle City Council today approved zoning in the South Lake Union neighborhood which would allow for office buildings as tall as 240 feet with floor plates as large at 24,000 SF. So how many stories is that? A good number to use is 14 ft per story. Based on those figures, a new building may be as tall as 17 stories and 408,000 SF.
For size/ height comparison purposes, you may want to take a look at Metropolitan Park East, which is located at 1730 Minor Ave. Seattle, WA 98101. Costar shows Met Park East is a 20 story building at 363,725 SF. Here are a couple of images:
For more details on the new zoning, please take a look at the announcement from the Seattle City Council News Release or at Mayor Mike McGinn's Blog.