Small tanks are much harder to maintain than large tanks because any mistake or problem becomes magnified due to the small volume of water in the tank. If your filter dies and toxic wastes start to accumulate, 30 gallons of water will dilute more and buy you more time than 5 gallons. Same goes for temperature, 30 gallons will maintain a stable temperature much better than 5 gallons.
30 gallons is the usuall reccomended starting size. The smallest I would go is 10 gallons, but only if you are very carefull with fish selection and maintanence. Keep in mind that only about 5-10 common species of fish can be kept in a tank that small.
Snails will not keep the aquarium clean. In fact, they add to the bioload (amount of life, which takes up resources) of the tank. They can be neat additions, but only if you specifically want them and have less fish as a result. Apple snails are particularly cool, but you need hard water and at least about 30 gallons to keep them.
Plants will help keep the tank clean, but you will need a filter. I would reccomend a power filter and that you avoid undergravel and air-powered filters. Aquaclear filters by Hagen are my personal favorite.
Do not get goldfish unless you have a big tank (about 55 gallons) and are ready for a lot of maitenence. They get big and make a lot of waste. I would reccomend schooling fish like tetras (get at least 6, 12 is betters, depending on size of tank) and maybe 1 or 2 larger peaceful fish like gouramies. Small catfish like corys would also be good.
Do your research- stores still sell many totally inappropriate fish like redtail catfish (tops out at about 3-4 feet) and bala sharks (1 foot, schools, huge tank). They also sell a lot of more advanced or difficult fish like cichlids.
Aquariums are not a huge amount of work, but they still require dedication and a lot of technical knowledge of things like water chemistry, the nitrogen cycle, mechanical troubleshooting for equipment, etc. All this stuff is available in aquarium books.
A smaller-sized aquarium would take about 10 minutes of work a day and about an hour on the weekends (weekly water changes are usually considered the best). If problems like filter issues or power outages occur, more time would be needed.
Feel free to ask me any more questions you have. I have about 5 years experience under my belt and 15+ tanks.