Traditionally, a katanna had a 28-30 inch blade and a wakizashi has an 18-20, sometimes less. I think your primary consideration should be balance, a sword should have a balance point that just in front of the guard, no more than a handspan in front. Ideally your off hand should be shorter and lighter as your off hand is likely to be weaker and less coordinated.
In most two handed styles the off hand weapon is primarily defensive, it takes the place of a shield. You see this in rapier\gauche, Florentine, tomahawk\longknife etc. There isn't a whole lot of technique written about Musashi's Go rin no sho but I've found that there are a few key points that apply not only in combat but life in general.
You are your own worst enemy, if a man can defeat himself his opponent(s) no longer matter.
Sloppy work is habit forming, style and habit make you predictable.
Use what is available to you. Musashi's swords weren't tucked in his obi for show, he had two so he used them.
Overwhelming aggression trumps skill. By not giving your opponent the chance to think you capture his mind.
I also recommend The Life Giving Sword by Yagyu Munenori. Munenori mastered a technique he called "throwing yourself under the sword" a maneuver other samurai of his day described as harrowing to watch and Munenori was the shogun's bodyguard and is said to have slain three men in the space of a breath.