The 300 or 320 ist the smallest of them all. I have a relatively big hand but I'm still going to get a 320 because I like = need = want another small pen for my T-shirt pocket. At least here, they both cost the same and the 350 is no longer being manufactured. The 300 and 350 nibs both have 14 K = 585 gold plus rhodium as does the 400 nib, but the 200's nib is "only" a gold-plated steel nib.
Lengths in cm, capped or posted are like this:
Souverän M400 -- 12.7 -- 14.9
Souverän M300 -- 11.0 -- 13.0
Souverän M200 -- 12.4 -- 14.7
Lots of luck
Mike
PS: tell your son/daughter that you need the pen as a present for your new grandchild