When use biggest and when use largest?

'Largest or Large' is aptly used with the quantity Words:

Large scale, Large amount, Largest Army, Largest city (considering area), Large quantity of, a large proportion, to a large extent', Largest Economies, Large volume, Largest group on face-book and Longest (as Identical).

'Biggest or Big' is one of most commonly used Word both in oral and written form:

As highest - Biggest building or biggest tower in the world.
As Important - Biggest decision of my life.
Many Fixed phrases - Big ideas, not quite a big deal, biggest mouth, very big.
Miscellaneous - The biggest changes in NASDAQ, Biggest Loss, Biggest Loser.

Hope this may help you!