Are the roses potted or in the ground? If some are potted, in what size pots are they? If potted, you have to be EXTREMELY careful applying this product as it can easily burn the plant in hot weather, particularly if they are the least bit water stressed.
If they are in the ground, why not try using a proportional amount on the smaller plants than on the larger? The label says 4 tablespoons in a quart of water treats one plant. That suggests a traditional sized plant to me, so if the smaller ones are one-third or smaller than the larger, try applying one-third to one-fourth the amount that you applied to the larger plant? You can always add more if needed, but you can't remove what's already been applied. You can't harm them by applying too little. I realize you haven't time at the moment to spend on the roses, but you also don't have time to try remedying a mistake, so I would simply water everything well, then apply proportional amounts of that quart of product solution based upon their relative sizes. Good luck and I hope what's eating your time and energy smooths out for you.
