Is this only a parking garage or is there an open lot too? I drive a large vehicle (over 8ft)

It is free parking. The station at Tukwila International Blvd also has a free parking lot.

No open lot

There are some spots that may take a large vehicle but very few

Open lot

It's a 6 story garage with over a thousand parking spots that fill up quickly during the week days. No open lot there.

It's a 6 story parking garage with no open lot.