Which is worse?

Getting a road bike that is a little small or getting a road bike that is a little big?

I am 5'8" and can't decide to get a 54cm or 56 cm bike. I am looking to start racing in triathlons.

I reckon 6 of one half a dozen of the other. Frame size within +/- 10mm can fit properly by selecting bars/stem etc...

If you go smaller, then the stem will be longer which might give you less twitchy steering (conversly, you might want a shorter stem & more direct steering).

The other consideration is that in a time trial set up, you'll be sitting further forward & specific time trial bikes have very short chainstays.

I think I've convinced myself you should be on a smaller frame...