I would factor in whether the joint was to be radiographed or not. If RT was required, it would be advisable to have the backing wider than the opening of the groove and width of the completed weld. It would make interpreting the film much easier for the film interpreter. If the joint was not examined by RT or if it was to be checked using UT, I would make the backing bar wider than the root opening, perhaps 3/4 inch plus the width of the root opening as a guide.
Best regards - Al