I like to use a break-down spear so that I can change the length depending on the conditions. My Crist has three 3' sections so that I can use it as a 6' or 9' spear. I also have 12" and 18" shafts so that the overall length can be changed from 7' to 10.5'. I also have a Ray Odor that has three 2' sections with both 12" and 30" shafts. The overall length on this can be 5' to 8.5'.
In a perfect world, I would also have a Gatku, but unfortunately it doesn't travel as well as the others do. If I lived in Florida and was diving locally, I'd have one.
To answer your question - Judah is right. If you're diving in the Bahamas with a 10' spear and you see a lobster, you'll wish it was 5'. If you're diving with a 5' and you see a Mutton, you'll wish it was 10'.