This idler for a Gates "CarbonDrive" bicycle belt is designed to take up slack -- it's not clear how well it would work as a tensioner (with significantly higher forces). The width of the outer (toothed) edge is 6 mm. The width of the center is 9 mm.
The idler is designed to take side-by-side 12 mm OD, 4 mm width bearings. As, for example, MR126-ZZ, here: http://www.fasteddybearings.com/6x12x4-bearings
Small lips at each edge of the center hole should provide a press fit.