As a rule, the ideal ratio between length and weight appears to be somewhere around 1 ounce per inch. That is, a 12 inch knife should weigh about 12 ounces. Such knives, however, while they can be fun to throw, are more or less useless from a survival viewpoint. I have a very small indoor throwing range in which I can perform half turn and full turn throws with very small knives and 6 inch nails. Location is another factor.
Larger knives require larger, thicker targets.
In my first year of throwing, my largest knives were about 11 inches long. Most of the time I was throwing things in the 8 to 10 inch range. My targets were made out of 2 inch thick Fir, and lasted months before I had to change out boards. I had to upgrade to much heavier 4 inch thick Fir. There is also, of course, cost.