You don't need to know the size. There is either a cone style filter that fits your car or there isn't. You can't make it fit if it doesn't. Ask a car place to see which filters fit your car. The filter should come with an adaptor that will attach to your oxygen sensor, then simply attach the cone filter to the adaptor and thats it. But you will have to custom make a mount because the mount is taken out with the stock airbox. Disconnect the negative battery terminal for 60min then reattach it. This resets the computer and lets it recalibrate. I am not sure if this step is necessary, but I did it anyway. Feel free to email me if you have anymore questions.