105 lbs and 5"3. Would 400-500cc would be too big for me?

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.   The size,
profile, and shape of the implant is based on your desired breast
size/shape, your chest wall measurements, and soft tissue quality. 
This decision should be based on a detailed discussion with
equal input from both you and your surgeon.   Make sure you
specifically look at before and after pictures of real patients who have
had this surgery performed by your surgeon and evaluate their
results.  
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

It is possible you have some features (e.g. very loose breast skin) that your consultant may be trying to address with a larger implant.  You would be well-served by obtaining consultation with another surgeon before proceeding with surgery.