There is no accurate way to predict what HRT will and won't do for a person, no matter their age. Everything has some degree of variability to it in any circumstance.
Now, being eighteen means you haven't likely masculinized too terribly much, compared to someone in their twenties, so you have that going for you. You won't grow more body hair, you won't get more masculine. Huge.
Body hair might be reduced, breasts will very probably grow, and as long as you don't get underweight, they should get to a good size - an A or a B is most common, but larger can be had.
I, myself, started at seventeen. It was a little disappointing for me, but I can't deny that looking at old pictures is hilarious because I simply don't recognize myself. Breasts? Maybe not - but then, I am practically a walking skeleton. A fact that I would love to repair, but am unable to.
Starting this young might mean a six, seven, or eight out of ten on your scale, depending on how masculinized a person was prior to starting HRT. Hormones don't reverse bone structure changes after all, and at eighteen they very rarely produce their own.
Perhaps watching some videos on Youtube would help. Keep me posted... keeping track of HRT changes is kinda fun.
