I would have to go with a 20 gallon. With this size they would have lots more options when it comes to stocking. I feel a tank this size is a good representation of what is required to be successful in this hobby. Once comfortable with taking care of a 20 a person new to the hobby should have the confidence to go up to any size. One could do the same with a 10 gallon but the stocking options are limited in a tank that small. I actually speak from experience since I started with a couple of 10's many many yeas ago.
I'm a beginner, and I started with a 40 gallon breeder. I like it. It's large enough to hold a good number of fish, has enough volume that water parameters won't get wonky too fast, and shallow enough that maintenance is fairly easy.
If money is the concern then imo a 55 is the best option, yes 20s maybe easier for water changes but a 55 is terrible. A 55 however provides enough water volume to cushion the blow of mistakes. A 20 gal has a much more volatile water parameters. And when new fish keepers go to the pet store and fall in love with the African cichlid or adorable oscar they arent harming/releasing that fish in 2 years because of how fast it grew. Working with a decent size canister filter also becomes an option at that point and will give a new fish keeper a much more hands on way of learning bio and mechanical filtration. And I also think that at 55gal cycling a new aquarium becomes necessary. This is more challenging then keeping a smaller tank but the knowledge gained from your first cycle is invaluable and necessary to truly care for fish properly
Definitely a 20 gallon. As others have said, alot more stocking options, but there is also the luxury of a larger tank. Things can go haywire very quickly with a 5 or 10 gallon tank. You have a little more leeway and margin for error with a 20 gallon.
I voted for 20 for the reasons many have stated. However, I am new and starting with a 40 and I like it. But it is big enough that I will be getting a python to make water changes easier. A 20 would be nice for a 25% change to be one five gallon bucket.
I started with a 55 gallon but after getting a 75 the extra width is nice so I would say a 75 gallon. 20 longs arent as intimidating and probably the smallest most versatile tank would be a 40 gallon breeder.
I voted 55G because I started with a 20 long and feel I would have had more success with a bigger tank.
I say a 55G because:
1. It's easy to overstock a 20G tank. After decor and substrate, you only have about 15 net gallons of water. About 7 or 8 danios and a few cories would overstock that. Algae explodes and fish aren't as healthy with the higher level of nitrate.
2. Cycling large tanks is easier if you do a fish in cycle like most beginners do. I didn't know about the cycle when I started, so imagine how hard it was for me (and my poor fish) when I learned about it and tried to cycle an overstocked 20 gallon tank...
3. More stable parameters
It's more work to do water changes and keep clean, but I think bigger is always better when it comes to aquariums. Even after I've been keeping fish for 5 years, I find my 75G tank much more stable and easier to maintain that my 14 gallon pea puffer tank and 8 gallon betta tank. It requires moving more water but everything else is easier.
I voted for a 10 gallon...more stable and versatile than a genuine nano tank, while still giving a decent number of stocking options and not being overwhelming to maintain. A twenty gallon is more stable still, but a lot of new hobbyists will not want to remove eight to ten gallons of water at once and then replace it with that much water every week. Plus, a lot of landlords will not allow you to have a 20 gallon (my apartment, for example, has a 20 gallon tank limit, so if I had a 20 gallon it would be the only indoor tank I could have), and a 10 gallon is less intimidating to landlords (and parents) paranoid about the tank leaking/breaking and ruining the floor/carpet/etc. Finally, a 10 gallon is easier to restart than a 20 gallon if (in the context of most beginners, when) the tank crashes.
I voted 20 gallon because its small enough for easier cleaning and water changes and yet big enough for a decent stock. I went back down to my 20 for awhile from my 55 just because it was easier. Then my 55 was staring at me one day and I put it back up Now I have a fully planted natural tank with ferts and all....Fishlore did not help with this in any way....
Maybe try a 29 gallon tank. My first tank was a 10 gallon and it just wasn't big enough for more than one or two fish. My 29 had a pair of convicts and a couple of catfish, and i had fish that kept me want more. Enter more tanks!
I would have said 20, but the trouble my brother has had with his 20 scared me away from that choice. I think 10 gallons is the perfect size to get someone "hooked" (get it ?). The size of a 10g is far more manageable than a 20, and a 10 gallon, when planted and stocked correctly, can match or even exceed the aesthetic appeal of a tank that is twice as large. In fact, the smaller size of the 10 would be a boon, since the larger size of the 20 would lead a beginner to believe the 20 can hold much larger fish than it actually can.
On the other hand, I would vote the 20 as the perfect second tank. Just big enough to satisfy the ambitions of a somewhat beginner, the possibilities for a 20 gallon are only limited by your imagination.
So, I would choose a 10 gallon as a starter tank every time. If you've had a little experience, a 20 gallon is the tank for you.
Annnnnd that concludes my review of 10 and 20 gallon tanks lol.
I'd say 30 to 50 gallon as still a manageable size but large enough to be stable and to offer some decent stocking options. Really though noob success depends on research. I've seen 15 gallons which I consider to be the smallest viable tank size for anything but a single male betta perhaps or some shrimp (and although 7 or 10 will work for those it sucks) be very successful with a well informed beginner. Likewise I've seen a beginner with good understanding of the hobby and science behind it take on a 100 or more gallon build successfully. At the LFS I work in we always encourage people to go as big as they can - not to make money as we don't many tanks or make an appreciable profit on the ones we do, but simply as it's going to be easier in the long run for the customer and better for their fish.