The average road bike handlebar size is 40cm. However, this can vary depending on the make and model of the bike. Some bike manufacturers will have different sized handlebars for different models. For example, a racing bike may have a smaller handlebar than a touring bike. There are also different widths of handlebars available. The most common widths are 38cm and 42cm. Again, this can vary depending on the manufacturer and model of the bike. If you are unsure about what size road bike handlebar you need, it is best to consult with your local bicycle shop or thebike manufacturer directly.