The smallest engine size that is commonly found on street motorcycles is typically around 125cc. Motorcycles with engine sizes in this range are often referred to as "small-displacement" or "entry-level" bikes. While the term "reasonable" can vary depending on individual preferences and riding conditions, a 125cc engine can be suitable for urban commuting and short-distance rides.
Small-displacement motorcycles are popular for several reasons. They tend to be more fuel-efficient, lightweight, and easier to maneuver in traffic compared to larger motorcycles. They are also generally more affordable, both in terms of the initial purchase price and ongoing maintenance costs.
It's worth noting that the power and performance of a motorcycle are not solely determined by engine size. Factors such as the motorcycle's weight, design, and technology can influence its overall performance. Additionally, legal restrictions and licensing requirements may vary by country or region, so it's important to familiarize yourself with the regulations in your area regarding engine size and licensing for motorcycles.
Ultimately, the choice of engine size depends on your riding needs, experience level, and personal preferences. If you're new to riding or primarily need a motorcycle for city commuting, a small-displacement bike like a 125cc can be a reasonable and practical option.