When I first got into road cycling way back in the early 90's, I ran a 39x23 low gear and felt foolish since so many others ran 42x21s. Since then, spinning has become the cadence of choice and 39x25s became more popular. Apparently you are a young and strong rider who can push the bigger gears. Although the smaller compact gears are probably not for you yet, you shouldn't go higher than 39x25 so you can save your knees for your next few decades of cycling.