The larger battery will give a larger reserve. If you are on shore power you are usually always charged up and starting will take very little juice (cranking amps) If you notice the charge meter you will see that after starting an engine (with only a few seconds of cranking) the meter will drop from 30+ amps to only a few amps, indicating that the battery is fully recharged.

If you are not on shore power and use the battery for other than starting (bilge pumps etc . . )a larger battery is a good idea.

No such thing as "too large" just takes longer to fully charge it.