The upright bass is a very valuable instrument to the bluegrass ensemble but it can be somewhat misunderstood at times.
While the electric bass guitar has been used in some bluegrass groups, the upright - or double bass is what's typical of the genre. Like other orchestral stringed instruments, basses come in fractional sizes. Unlike the violin counter parts, in the United States a "full-sized" bass is actually a 3/4 size bass. It is extremely rare to encounter even a 7/8 bass. In bluegrass music, the bass is traditionally played without a bow - by plucking the strings with bare fingers. It is well worth investing a little bit of money into having it set up by a professional as many of the more affordable options are shipped in less that ideal playing condition.
Palatino VB-004 3/4 Size Upright Bass - This bass is made out of spruce and maple plywoods with ebony fittings. This is a great option for jam sessions where an instrument of this size is bound to take some bumps. The outline of this bass is gamba shape, rather than a violin shape. Again, the lack of delicate corners is helpful in crowded company.