rule of thumb for the lumber needed for any given span is height times height times width. (nominal)
2x4 = 2x4x4 = 32" span (3')
2x6 = 2x6x6 = 72" span (6')
2x8 = 2x8x8 = 128" span (10' 8")
2x10 = 2x10x10 = 200" span (16' 8")
It is just a rule of thumb and at the extremes it is faulty but from 2x6 to 2x12 it is close.
________
Honda Vfr1200F