Wickers, A 15x8 prop is one that is 15 inches long with 8 degrees of pitch. The bigger the prop or the more pitch, the faster it will pull you through the air. You can't just stick on a bigger prop (length or pitch). You need to know what the engine can handle or "turn". You do not want the engine overloaded and struggling. You also do not want to put on a prop that is too small. You will over rev and damage the engine. I am not familiar with your engine so take the previous advice and keep an eye on the engine. Go up or down on the prop to adjust your rpm's into the appropriate range. Be careful when using a longer prop, watch for ground clearance and your fingers.
Keep in mind!! This is over simplified. You need to start with the recommended prop then experiment. Some prop combo's give speed some give more torque, etc.