Both your height and weight should play an important role when you are choosing a skateboard. You need to find a skateboard, which fits your weight and height. Picking one, which is either too small or too big might only cause you to experience difficulty controlling it, especially if you are still a beginner learning the ins and outs of skateboarding.
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.