The best way to increase the size of your delts is to primarily work in the hypertrophy range of 8-12 reps. However, you should also include some more strength movements as increasing strength will allow you to do more volume. Therefore, you should have 1-2 compound exercises that hit your rear delts using heavy loads in the 4-6 rep range. Then use 2-4 assistance or isolation movements in the 8-12 rep range. You should aim to train your rear delts twice a week