What is the brightest scale?
The brightest note of the brightest mode, Lydian, is the 11. Interestingly, that note is also the darkest note (the 5) of darkest mode, Locrian.
What is the largest possible chord?
Typically, chords are built in 3rds and the biggest chord I know is a 13th chord which can have seven notes in it, but usually have some note omitted because of it's size (the 9th and the 11th are optional).