Chain length matters.

If your chain is too short, and you ever try to shift into the big/big combination, it can cause serious damage to your bike.
If your chain is too short, and you shift into the little/little combination, the chain will go slack on the return run.

Unless I'm POSITIVE the old chain was the correct length, I always resize a new chain. It only takes a minute or two.
1. Wrap the new chain around the biggest front chainring and biggest rear cog bypassing the derailleur.
2. Figure the shortest place you could rejoin the chain (hint: there's wide and narrow links).
3. Add one inch of chain (2 links or 1 narrow link and 1 master link.
That will yield the shortest chain that will work with your gear combination and the length of the derailleur cage doesn't matter.