In general an axle width should be chosen that is close to the width of the deck it will be used with. For example, a 7.75" wide deck will usually be fitted with trucks that have axles between 7.5" wide and 8.0" wide. (Standard truck axle nut size is 5/16"-24tpi UNF, and the thinner "jam" style with an optional nylon lock.) Trucks that are too wide can make doing tricks difficult and can cause the wheels to get in the way when the skateboard is being ridden. Trucks that are too small can be hard to maintain stability and can cause wheel bite to occur when turning.
To get higher ollies, I would suggesting resting your back foot on the very end of the tail and the front foot in the middle of the deck to allow for more slide. Popping as hard as you can and catching your board in the groove of the nose with you ankle practically resting in the groove will level the board out in the air. after this initial movement slide you feet back to the bolts on the tail and nose to ensure a clean landing.