A beginner is more likely to make mistakes than when some experience has been gained. That is true for most things including keeping a marine aquarium.
A larger aquarium, as large as can be fitted and afforded, has always been the normal advice to a beginner. This is because if the aquarist makes an error with, say, an additive there is more seawater to absorb the error. Also, if the fish are overfed, which is a common ands understandable mistake, the effect on the seawater will not be so severe so quickly. Routine water changes should be being done, but nevertheless water quality would suffer. In addition, a larger volume of water is less subject to variations in outside temperatures. If warm weather arrives then a small amount of water will heat up more quickly than a larger amount.
As Peter once put it, a spoonful of sugar in a cup has more effect than a spoonful in a bucket.
Overall, I suppose the larger aquarium is still the better choice? I am no longer so sure. Surely no aquarist is going to apply an additive to the aquarium without reading the instructions to obtain the application dose rate. If overfeeding occurs, then a small aquarium will have less fish than a larger one and so the danger from overfeeding would be evened out maybe. The question of water temperature increase hasn’t changed of course.
There are advantages to the smaller aquarium. A beginner may be tempted to skip routine water changes now and again, as they take preparation and need a fair bit of salt for a larger tank. A Nano is easy, as the gallonage involved is small, so there’s no problem with preparation, storage or application.
Another thing about the small aquarium is that it is cheaper to set up. If the beginning aquarist wants to see how it goes for a while then the outlay is minimised. Also there is less to look at, so attention is drawn to the fewer livestock in the tank and any problems are more likely to be seen.
So, as said, I have mixed feelings. There are many beautiful tanks, big and small. Some of the big ones are stunning in their impact. Many small ones are beautiful in their detail. Also many new aquarists do keep small aquariums and succeed.
So, if anyone asks me ‘What size aquarium should I go for?’ my response is likely to be two questions: how much room have you available, and what do you fancy?
Is the ‘largest tank’ advice out of date?