i know this isn't about goldfish, but i was wondering what's the smallest tank needed to support just one or two small marine invertebrates with maybe a small chunk of live rock.

People have done very very small tanks, but they are much harder to keep everything in balance and not recommended for a beginner. Little things like being just slightly off with the salinity after a water change can crash the entire tank, because it takes so much less salt to make a big change in a little tank. Make sense? Most people don't suggest trying anything less than a 55g for your "first" marine tank just because its way too easy to screw some little thing up and kill all your inhabitants.

Do a web search on "nano reef" which is the term used for small reef setups. A great place to get info on marine tanks is www.wetwebmedia.com

i think the same applies to ALL TANKS (whatver the inhabitants, brackish, saltwater, goldfish, freshwater, marine, reef etc.....)

the BIGGER the tank...
the MORE FORGIVING it will be if you make a mistake.
the water parameters will more MORE STABLE.
mistakes will have LESS EFFECT