For the Boxford the standard sizes are 5 in 3 jaw and 6 in 4 jaw with anything bigger held on the 9 in faceplate. However it is nice to have smaller lighter weight chucks that can be easily transferred onto a small dividing head or rotary table when doing smaller work.
Having a bigger chuck does not make any part of your anatomy grow bigger. If you need to hold bigger items frequently you probably need a bigger lathe.