The paper report results of magneto-thermoelectric properties the single crystal Bi_1-xSb_x nanowires in glass cover with different diameters prepared by Ulitovsky-Taylor method. We study the effect of nanowire dimensionality in the Bi_1-xSb_x nanowires in semimetal and gapless states. In the semimetal region Bi_1-xSb_x nanowires (x = 0.02) the semimetal-semiconductor transition due to quantum size effect at the critical diameter significantly higher than for pure Bi-wire are observed. This allowed to separate the effects related to the size quantization and surface state. It is shown that a weak magnetic fields leads to an increase of the hole contribution to the thermopower and to increase the Power factor α²σ at positive α in Bi-2 at% Sb nanowires. Bi _1-xSb_x wires in gapless region (x = 0.06) shows the increase of the energy gap with decreasing the diameter d and increase of the electron contribution to the thermopower ay in weak transverse magnetic field at the negative α. This will use the investigated wires as n- and p-branches in thermoelectric energy converters. Power factor depending on diameter Bi _1-xSb_x nanowires, composition, temperature, magnetic field is calculated.