Looking at page 7 in the link Dick gave, there seems to be a radial hole on the top of the stud the roller rides on. That leads me to think there may be an axial hole down the stud, so you are supposed to force oil down the axial hole somehow, and thence into the radial one to lube a plain bushed bearing. On page 8, there's no mention of a roller bearing in the parts list.
The size of the tensioning roller shouldn't matter too much. I'd make it fairly large, to reduce its speed of rotation, and (if it bears on the back of the belt) to reduce the reverse curve it gives to the belt. It would do no harm to fit a ball bearing, either.
Andy