As a general rule, an experienced sailor will be able to handle a boat less than 28 feet or 8 meters without systems and less than 4 feet or 14 meters if with systems. You can sail alone, but if your boat manufacturer has made provisions for a crew cabin, it is a good idea to take one with you.
Note that length is not an issue when it comes to sailing with a part crew. The only limitation is your ability to handle the amount of sailcloth safely. It all depends on your experience and ability. Another consideration is stability. A larger boat tends to be more stable and can be controlled from the cockpit. Compared to a smaller boat, a larger one may be easier to steer by one person. However, you have to be careful when docking since you are sailing a large boat.