My daughter is 21 mos old but was born at 24 weeks of pregancey she had a MRI done the other day and said that her corpus callosum appears slightly sm?

Normal variation. A major problem with MRI is that it often shows useless information. Severe premature babies can have delayed myelination that is of no clinical significance. I would ignore the MRI and just enjoy how well she is trying to recover from a very early birth. I do not approve of using the MRI to try to predict outcome.