12x6 is the minimum required.
Check with the AHJ, some jurisdictions require that 1500psf be assumed unless supporting geotech indicates a greater bearing value may be used, which would require 15x6 footings..
As a cautionary note, the minimum footing widths established under Table R403.1 are only based upon 20-ft of roof tributary at 50-psf snow load, 16-ft of floor tributary, 10-ft first story walls, and 8-ft second story walls. If your design uses spans imparting greater tributary, footings may need to be increased.
This is reflected in the 2015 IRC by footnote "b" of Table 403.1(1). Per the 2015 IRC, footings follow the table, then are increased at a prescribed rate of 2-in to width and 1-in to depth for every 2-ft of building width over an assumed width of 32-ft.
As an example, a 40-ft wide home with a center load bearing wall of conventional 2-story construction would require 24x10 footings (16x6 typ. + 8-ft adjustment [8x4] = 24x10).
Generally speaking, following footnote "b" of the 2015 IRC will greatly oversize footings because it is based upon a floor and roof system bearing on a center load bearing wall. Thus, this footing is sized for 50% of all roof and floor loads. Given the 40-ft example home, it is infeasible to assume that the floor system would clear span 20-ft to the center wall, whereas roof system is manageable with trusses.
