The larger the tank, the better, but you need to weigh this against the space you have available. As a minimum most people can find space for a 20 gallon tank. Tanks are either glass or acrylic. While many people favor the former, if you have children, acrylic is probably a safer option as it won’t break. Tanks are heavy so you need to plan where you will place it.
In addition, acrylic tanks need support all the way along the bottom, not just at the edges.
Your aquarium will need a lid; sometimes these come with a light already fitted. A lid is needed to slow evaporation, and to prevent fish making a bid for freedom.