Built track bikes like the Felt T3 typically come with 48/15. beginners do well with a 48/16. If you are in good shape, 48/14 can work too.

Ideally you would have some practice/training sessions and get your comfort level with the track, bike, gearing. We are talking ~$20 for a cog. Get both the 14 and the 15 and see which one you want.