The contrast of syllables makes it flow better. Ideally, the middle, first, and last name should all have a different number of syllables. If your last name is monosyllabic, you should go with a longer middle name. For example:
Charlotte Elizabeth Day
Agnes Victoria Lee
If your last name has 2 syllables, a monosyllabic middle name would sound best:
Charlotte Ann Walters
Agnes Rose Cowan
That said, I would go with a fun middle name if I could. I like the idea of playing with the middle name, since it is really just a space holder. I gave one dd the middle name Maranatha, which has religious meaning. I wanted to give dd#1 the middle name Bluebird, but dh vetoed it.