It will feel large - no, huge! - compared to your vintage dress watches however you'll adapt, and you're correct in stating that the 5xxx series is the baby of the (adult) G family. Great watches though, I've a 5610 and a 5000 as I get a kick out of the nerdy solar and atomic functions, I'm wearing the 5610 right now actually as it's normally what I have on when knocking about at home.
If you think that the 5xxx is big get a GX56 on your wrist!
Edit to add that I didn't realise that the 5600 was certified by NASA, as if I need another nerd reason for buying another watch...