At 5'11" and 140 pounds, you are very tall and thin. Many of us are extremely jealous of your size and weight.
Assuming that your Testosterone levels were normal pre-HRT, your genetics are mainly responsible for your size and weight. Estrogens will just make your muscle mass diminish even more. Serious strength training will help some, but to put on muscle, you need Testosterone. There is no way around that.
If your first priority is feminization, then you need to be between 20% and 30% body fat, like most women are. I would suggest a good balanced diet and moderate strength training with some aerobic and stretching mixed into your daily workout. Just let the hormones do their magic. Many women can become stronger than they expected by good diet and exercise.