28mm clearance is the current standard for modern road bikes and works well if you stick to smooth pavement. It accommodates the most popular road tire width and gives you a solid balance of speed and comfort for typical road riding, club rides, and racing.
However, 28mm is really the minimum you should accept on a new road bike today. If your local roads are rough, chip-sealed, or you ever want to dabble in light gravel, you'll likely want 30-32mm tires for the extra comfort and grip. The key thing to remember: you can always run narrower tires on a bike with wider clearance, but you can't squeeze bigger tires into a tight frame.
Also factor in that you need 3-4mm of space between tire and frame for mud and wheel flex. So a bike with 28mm max clearance realistically means running 25-26mm tires with proper safety margins.