Ooo, one of my favorite topics, tank dividers!
Based on my own experience, a 5.5 is perfectly find to divide in half for a pair of long-finned males. I have two 5.5s that have both been set up divided for quite some time without much trouble at all. I wouldn't divide it any more then in half, or divide anything smaller, but I feel split in half a 5.5 still provides adequate room.
As far as the actual divider itself goes, I follow the instructions here with a little twist of my own.
MUCH cheaper and more stable then anything you'll find pre-made in a store anyway, I can assure you. But, I instead double up on the craft mesh and do a little more exact measuring and trimming so things fit nice and snug without that bend. Doubling up on the mesh gives just a tad but more stability to the whole thing and I've found it REALLY helps with the issue of males being able to see each other though the dividers. After putting several males in several different tanks made with the doubled-mesh dividers I noticed significantly less flaring and attention paid to their tankmates then with just a single-mesh divider and I've had far FAR less divider mishaps since I started doubling up on mesh.
Hope that helps some! And good luck with whatever you decide on doing!