There is no real way to tell how big a baby will be, what your midwife has however done is given an approximate weight of an "average" sized baby, so chances are she might be right. My daughter was 9lb 3oz no one even suggested that she would be big, however after she was born my student midwife said that she thought she was on the large size, lol.
7-8lb would be a good size, good luck, hope it all goes well.