I would forget about all the beams, and simply span the 21 feet with
2x12's
at 16" OC, and use 2x6 rafters at 16" OC. Nail the rafters to a 2x8 ridge
board at the peak.

My shop is above my two-car garage. I went with the Beam and 2x8's as
suggested by Claudio. The only difference was that I used steel. YMMV, but
my 12"x25' steel beam cost around $260 and about the same for the guy with a
crane to deliver and drop it into place. The cost of the beam was offset by
being able to use 2x8s to frame the floor. I would have otherwise needed a
nominal 14" floor joist to span the whole garage. The bonus was that I got a
free 6" of headroom upstairs bu virtue of the thinner floor.

It sounds as though elevation limiting factor. A beam approach can buy some
extra head room.

By the way, it's a very solid floor.

-Steve