I have three nano systems in my office. They all happen to be 24 gallons. There is no best system. People have don't some great things in tiny pico tanks and 12 gallon systems,
That said, I just like the little extra galonage with a 24 for slightly better parameter stabilty and more coral/fish choices.
Don't rely entirely on others and this forum. Make the small investment in some good books then supplement with the information that other reefers generously provide.
Choices: Jbh Nano deluxe, aquapod, new oceanic biocubes. all reasonable choice and I am sure you will be bombarded with every bodies opinions. Don't get yourself even more confused about lightning. The lighting with these systems will all support fish and the type of lower level lighting corals you will want to have and of course, "Keep the peace".