Welcome to R2R! A larger tank is more stabile and it takes longer for things to go wrong but I actually think a beginner is better off with a smallish, but not too small, tank as a first foray into saltwater. I'd say anywhere between 10 gallons and 40 gallons is a good first tank. Big enough to have a little stability but small enough that water changes, scaping, etc. are not so intimidating.
Welcome to R2R as a 20+ year Freshwater pro, you've come to the right place!
So, mistakes in this hobby are far more expensive because livestock is more expensive. Large volumes of water fluctuate parameters slower which is always better. Unlike your freshwater tanks where topping off your aquarium with fresh water was mainly for looks, keeping your tank topped off with freshwater on a salt tank keeps the tank at optimal salinity levels. If 1 gallon of water evaporates out of a 75 gallon tank, it's not nearly as bad as 1 gallon evaporating out of a 14 gallon tank. Your salinity swings on the smaller tank will be monumentally more challenging to control without an Auto-Top off system. Whereas with a 75 gallon system, it doesn't necessarily need one right away. Also, one of the things you need to master are food/portion control so you can keep nitrates and phosphates down, again, this is much easier to accomplish in a 75 gallon tank because you can put in a little too much food every once in a while and it will be more forgiving, not so in a small tank.
