As said, goldfish are not small, yes you get babies but they should grow large, up to and excess of a foot long depending on type. Only stunting their growth will keep them small, and it isnt healthy for them. Guppys, small tetras, small rasboras can be kept in smalle tanks but they need to be in groups so larger bodies of water are far better for them. A siamese fighter can do ok in a smaller tank, but will use the space if given larger so keeping them in smaller does seem an awful thing to do to me. Also, the larger the volume of water, the easier it is to keep the water healthy for the fish.
Whatever fish you want to keep, no matter how small, the water will need to have a filter running in it 24/7 so that itself will limit the minimum depth you have.