Theres a balance with all 3 dimensions. Ultimately it depends on the fish and your maximum space. Width should be at least wide enough for the fish at full size to be able to turn around with ease. Height should be tall enough so that full grown fish can swim above and below each other without colliding. Finally, length [in my opinion] should be at least 2.5 to 3 times the full grown length of the fish but should be as long as you can afford. In the end all dimensions are important and should be as large as you can afford, but, length is what will give the fish the most room to swim and exercise so that should be the dimensions you try to extend as far as you can assuming all other needs are met.