Fortunately I could expand my box another 100 spaces and be perfectly fine so it hasn't become an issue... yet.
On a side note, I hit day 250 today so I'll post my box progress. While you may notice the distinct lack of any legendaries, do take note of my exquisite complete Chopper evolution collection with rare units like the elusive Heavy Point Chopper. Thanks rare recruit!
Nah, you have a great box. Congrats! I don't have a Legendary yet either.
As for farming, I might have over-farmed. Just a bit.
Halloween Island:
Skillbook
Ace - 22 [current skill level 2]
Marco - 25 [current skill level 4]
Jozu - 12 [don't have]
Impact Usopp - 36 [current skill level 4]
Rayleigh - 8 [don't have]
Units
Halloween Robin - 13 [current skill level 7]
Halloween Brook - 10 [current skill level 5]
Halloween Sanji - 14 [current skill level 3]
Halloween Usopp - 7 [current skill level 4]
Halloween Zoro - 32 [current skill level 1]
Coby Island:
Skillbook
Robin - 1 [current skill level 3]
Veau Shot Sanji - 9 [current skill level 3]
Helmeppo V2 - 5 [current skill level 3]
Coby V2 - 10 [current skill level 5]
So yeah, I'm going to free up A LOT of space the next time a skill-up event happens. If I need to, I'll get rid of the Veau Shot Sanji/Jozu skillbook, so the first isn't really needed and I don't have the latter.
I'm debating whether I should move on and try for Coby/Helmeppo V2 and Robin skillbooks. I know Robin's skillbook will be making a return in a newer island soon, hopefully with a much better drop-rate. As for Helpmeppo/Coby V2, the former is that useful of a unit, and Coby V2 is already at 13 turns, that sync's perfectly with a max skill Garp]. I'm not sure what I'll do.