There is no definitive answer to this question as it depends on a number of factors, such as the rider's height and build, the type of bike they are riding and the terrain they will be riding on. However, generally speaking, a TT bike should be smaller than a road bike. This is because TT bikes are designed for speed and aerodynamics, and so having a smaller frame helps to reduce drag. Additionally, riders who are racing against the clock often prefer a smaller bike as it is easier to handle at high speeds.