Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
A tank under 20 gallons tends to be unstable, the water parameters can change too fast for a beginner to keep up with.
Much larger than a 20-29 gallon and it might be too much for a beginner, financially, or otherwise.
I like recommending a standard 20 or 29 gallon tank for a beginner. Big enough that they can start slowly and build to something nice. Big enough that they can think about more species of fish than would be happy in a 10 gallon tank. Big enough that the volume of water acts as a sort of buffer to keep things more stable. Problems build more slowly, so the beginner has more of a chance to notice that there may be a problem.
Still not too much money to lay out getting started, but yes, a respectable sum. Perhaps enough that the beginner will be a bit more cautious and do a little research and have a better chance of success.