"Small or big ? Bowl for a start ? Will a bigger be more difficult to maintain?"
This is a very important question and decision making when you are starting out the hobby. With my years of experience I have seen many people take wrong decisions on this part. I many times see people buying a bowl for example, which is just not a good decision. Let me explain
Fish keeping in bowls is inhuman for several reasons, first an foremost there is not enough space, low surface to air ratio and no filters to clean the water and or even worst case scenario there is no oxygen supply. Therefore you should never even think of keeping a fish in a bowl, you are simply waiting to kill your fish. Its complete inhumane. I would also like to add that I have seen in many places that they are selling goldfish in a bowl ? Never ever buy anything like that.
1. Nano Aquarium upto 1.5 ft / 18’’/ 45 cm
Nano aquariums are good if you a space problem and just cannot go beyond nano aquariums of 18’’. They are also good if you are looking to start small. Nano aquariums can fit into any space and are specially good for keeping the nano fish varieties such as boraras briggatte or even a beautiful betta. You can also maintain a low tech semi planted nano aquariums with plants such as anubias with least maintenance. I also find nano tank specially good for college and school students who are tight on budget and want to start the hobby, this will teach them a lot. Never put larger fishes such as angles oscars or even large tetra species in a nano aquarium.
2. Medium size tanks upto 3 ft / 36’'/ 90 cm
This is the best suitable size to start your aquarium hobby with, the bigger the tank the easier to maintain and the bigger margin for error for you to when starting. More over when compared to cost for example the light price will be with just a difference of about 20% between 2 feet or 3 feet, even a canister price will have the same margin. Moreover another advantage of starting with medium size is when you upgrade to a larger aquarium for example 6 feet then you can use a 3 feet light of your present tank and just buy another 3 feet hence 50% discount for your upgrade you are getting, same applies to a filter.
3. Large size tanks 4 ft / 48’’/ 120cm and above
They are also choice of many planted professionals as they get more depth in the scape.The larger aquarium are specially suitable for those who want to keep specific fishes such as certain cichlids or monster fishes. If you have the space to accommodate a larger tank then the larger aquarium is always the best buy as its going to be the last aquarium you are ever going to buy (maybe !), there is no upgrade or course you can buy another one, but you can start to invest into the best equipments with a forecasting future. And as the thumb rule says, the bigger the better the more balance of nature.
The best aquarium to start your hobby should be Medium size tanks upto 3 ft / 36'’/ 90 cm as there is one thumb rule, the bigger the aquarium the water chemistry and temperature will stay more stable, and more balanced and you will have more room for error. For example I want to imagine you aquarium as a glass of water. A nano tank will get hot faster as compared to medium or larger aquarium and vice versa. If there is an algae issue in a planted aquarium, you will within a day or two you will have algae covered in nano aquariums where as medium and large aquarium will take longer time hence more time in hand to correct things. Think of aquarium in a nature as balanced ecosystem, the bigger the better.
THE FISH APPROACH !
Many people when starting out take the fish approach, for example if you have decided to keep a Arowana, then a 3 feet aquarium would become small within few months and you would have to upgrade to a large aquarium for example a 6 feet with more depth as arowana would need space to turn. I would say for a fish approach the first thing should be researching the fish online and then deciding the aquarium size. Another method is to see some long term videos on the fish over youtube and how they keep them in long term. Be sure to check the maximum size the fish can get.