To shorten half halt with your leg and rein whilst saying "woah" To lengthen use your leg to move him forward into the bit! I hope this was helpful!
I half halt with my leg on but I'm also trying to use my seat more
You use your hips to control the canter. So for a shorter stride youll make the circle smaller and for a longer stride you make the cirle larger but keep the speed the same. For a trot if you sit it, allow for your hips to follow the horse. Use your abs to pull your belt buckle up, almost as if you have an imaginary person pulling it up