TL:DR Enlarge person does NOT affect an Unseen Servant, nor anything that the Unseen Servant is carrying.
Right or Wrong.
Long Version.
I am a 4th level Wizard with a Heavy Crossbow. I purchase Large Sized Crossbow Bolts (From Baubs Crossbow Hut).
One morning, out in the field. I cast Unseen Servant, and I name him James. After Breakfast, I hand James the quiiver of Large Crossbow bolts to carry, twlling James to hold these out for me.
Some time later (while James is still active), My party is attacked. I cast Enlarge Person on myself. My Crossbow gets Large. James, however, does NOT change size, and nothing Jame is carrying is enlarged. SO, My large Crossbow Bolts are Still the perfect size for me to use, AND, Crucially, easily accessible to me. I can take them from James and reload the crossbow with them (I have Rapid Reload for heavy Crossbow, so Move action).
Hilarity Ensues.
(For the record, the ideal state is to combine this with Gravity Bow, and True Strike. +20 to hit, with 3d8 damage per, and a 19-20 crit range. Not bad for a 1st level wizard, No?).